allennlp.nn.util¶
Assorted utilities for working with neural networks in AllenNLP.

allennlp.nn.util.
add_positional_features
(tensor:torch.Tensor, min_timescale:float=1.0, max_timescale:float=10000.0)[source]¶ Implements the frequencybased positional encoding described in Attention is all you Need .
Adds sinusoids of different frequencies to a
Tensor
. A sinusoid of a different frequency and phase is added to each dimension of the inputTensor
. This allows the attention heads to use absolute and relative positions.The number of timescales is equal to hidden_dim / 2 within the range (min_timescale, max_timescale). For each timescale, the two sinusoidal signals sin(timestep / timescale) and cos(timestep / timescale) are generated and concatenated along the hidden_dim dimension.
 Parameters
 tensor
torch.Tensor
a Tensor with shape (batch_size, timesteps, hidden_dim).
 min_timescale
float
, optional (default = 1.0) The smallest timescale to use.
 max_timescale
float
, optional (default = 1.0e4) The largest timescale to use.
 tensor
 Returns
 The input tensor augmented with the sinusoidal frequencies.

allennlp.nn.util.
add_sentence_boundary_token_ids
(tensor:torch.Tensor, mask:torch.Tensor, sentence_begin_token:Any, sentence_end_token:Any) → Tuple[torch.Tensor, torch.Tensor][source]¶ Add begin/end of sentence tokens to the batch of sentences. Given a batch of sentences with size
(batch_size, timesteps)
or(batch_size, timesteps, dim)
this returns a tensor of shape(batch_size, timesteps + 2)
or(batch_size, timesteps + 2, dim)
respectively.Returns both the new tensor and updated mask.
 Parameters
 tensor
torch.Tensor
A tensor of shape
(batch_size, timesteps)
or(batch_size, timesteps, dim)
 mask
torch.Tensor
A tensor of shape
(batch_size, timesteps)
 sentence_begin_token: Any (anything that can be broadcast in torch for assignment)
For 2D input, a scalar with the <S> id. For 3D input, a tensor with length dim.
 sentence_end_token: Any (anything that can be broadcast in torch for assignment)
For 2D input, a scalar with the </S> id. For 3D input, a tensor with length dim.
 tensor
 Returns
 tensor_with_boundary_tokens
torch.Tensor
The tensor with the appended and prepended boundary tokens. If the input was 2D, it has shape (batch_size, timesteps + 2) and if the input was 3D, it has shape (batch_size, timesteps + 2, dim).
 new_mask
torch.Tensor
The new mask for the tensor, taking into account the appended tokens marking the beginning and end of the sentence.
 tensor_with_boundary_tokens

allennlp.nn.util.
batch_tensor_dicts
(tensor_dicts:List[Dict[str, torch.Tensor]], remove_trailing_dimension:bool=False) → Dict[str, torch.Tensor][source]¶ Takes a list of tensor dictionaries, where each dictionary is assumed to have matching keys, and returns a single dictionary with all tensors with the same key batched together.
 Parameters
 tensor_dicts
List[Dict[str, torch.Tensor]]
The list of tensor dictionaries to batch.
 remove_trailing_dimension
bool
If
True
, we will check for a trailing dimension of size 1 on the tensors that are being batched, and remove it if we find it.
 tensor_dicts

allennlp.nn.util.
batched_index_select
(target:torch.Tensor, indices:torch.LongTensor, flattened_indices:Union[torch.LongTensor, NoneType]=None) → torch.Tensor[source]¶ The given
indices
of size(batch_size, d_1, ..., d_n)
indexes into the sequence dimension (dimension 2) of the target, which has size(batch_size, sequence_length, embedding_size)
.This function returns selected values in the target with respect to the provided indices, which have size
(batch_size, d_1, ..., d_n, embedding_size)
. This can use the optionally precomputedflattened_indices()
with size(batch_size * d_1 * ... * d_n)
if given.An example use case of this function is looking up the start and end indices of spans in a sequence tensor. This is used in the
CoreferenceResolver
. Model to select contextual word representations corresponding to the start and end indices of mentions. The key reason this can’t be done with basic torch functions is that we want to be able to use lookup tensors with an arbitrary number of dimensions (for example, in the coref model, we don’t know apriori how many spans we are looking up). Parameters
 target
torch.Tensor
, required. A 3 dimensional tensor of shape (batch_size, sequence_length, embedding_size). This is the tensor to be indexed.
 indices
torch.LongTensor
A tensor of shape (batch_size, …), where each element is an index into the
sequence_length
dimension of thetarget
tensor. flattened_indicesOptional[torch.Tensor], optional (default = None)
An optional tensor representing the result of calling :func:~`flatten_and_batch_shift_indices` on
indices
. This is helpful in the case that the indices can be flattened once and cached for many batch lookups.
 target
 Returns
 selected_targets
torch.Tensor
A tensor with shape [indices.size(), target.size(1)] representing the embedded indices extracted from the batch flattened target tensor.
 selected_targets

allennlp.nn.util.
bucket_values
(distances:torch.Tensor, num_identity_buckets:int=4, num_total_buckets:int=10) → torch.Tensor[source]¶ Places the given values (designed for distances) into
num_total_buckets``semilogscale buckets, with ``num_identity_buckets
of these capturing single values.The default settings will bucket values into the following buckets: [0, 1, 2, 3, 4, 57, 815, 1631, 3263, 64+].
 Parameters
 distances
torch.Tensor
, required. A Tensor of any size, to be bucketed.
 num_identity_buckets: int, optional (default = 4).
The number of identity buckets (those only holding a single value).
 num_total_bucketsint, (default = 10)
The total number of buckets to bucket values into.
 distances
 Returns
 A tensor of the same shape as the input, containing the indices of the buckets
 the values were placed in.

allennlp.nn.util.
clamp_tensor
(tensor, minimum, maximum)[source]¶ Supports sparse and dense tensors. Returns a tensor with values clamped between the provided minimum and maximum, without modifying the original tensor.

allennlp.nn.util.
clone
(module:torch.nn.modules.module.Module, num_copies:int) → torch.nn.modules.container.ModuleList[source]¶ Produce N identical layers.

allennlp.nn.util.
combine_initial_dims
(tensor:torch.Tensor) → torch.Tensor[source]¶ Given a (possibly higher order) tensor of ids with shape (d1, …, dn, sequence_length) Return a view that’s (d1 * … * dn, sequence_length). If original tensor is 1d or 2d, return it as is.

allennlp.nn.util.
combine_tensors
(combination:str, tensors:List[torch.Tensor]) → torch.Tensor[source]¶ Combines a list of tensors using elementwise operations and concatenation, specified by a
combination
string. The string refers to (1indexed) positions in the input tensor list, and looks like"1,2,1+2,31"
.We allow the following kinds of combinations:
x
,x*y
,x+y
,xy
, andx/y
, wherex
andy
are positive integers less than or equal tolen(tensors)
. Each of the binary operations is performed elementwise. You can give as many combinations as you want in thecombination
string. For example, for the input string"1,2,1*2"
, the result would be[1;2;1*2]
, as you would expect, where[;]
is concatenation along the last dimension.If you have a fixed, known way to combine tensors that you use in a model, you should probably just use something like
torch.cat([x_tensor, y_tensor, x_tensor * y_tensor])
. This function adds some complexity that is only necessary if you want the specific combination used to be configurable.If you want to do any elementwise operations, the tensors involved in each elementwise operation must have the same shape.
This function also accepts
x
andy
in place of1
and2
in the combination string.

allennlp.nn.util.
combine_tensors_and_multiply
(combination:str, tensors:List[torch.Tensor], weights:torch.nn.parameter.Parameter) → torch.Tensor[source]¶ Like
combine_tensors()
, but does a weighted (linear) multiplication while combining. This is a separate function fromcombine_tensors
because we try to avoid instantiating large intermediate tensors during the combination, which is possible because we know that we’re going to be multiplying by a weight vector in the end. Parameters
 combination
str
Same as in
combine_tensors()
 tensors
List[torch.Tensor]
A list of tensors to combine, where the integers in the
combination
are (1indexed) positions in this list of tensors. These tensors are all expected to have either three or four dimensions, with the final dimension being an embedding. If there are four dimensions, one of them must have length 1. weights
torch.nn.Parameter
A vector of weights to use for the combinations. This should have shape (combined_dim,), as calculated by
get_combined_dim()
.
 combination

allennlp.nn.util.
device_mapping
(cuda_device:int)[source]¶ In order to torch.load() a GPUtrained model onto a CPU (or specific GPU), you have to supply a map_location function. Call this with the desired cuda_device to get the function that torch.load() needs.

allennlp.nn.util.
find_embedding_layer
(model:torch.nn.modules.module.Module) → torch.nn.modules.module.Module[source]¶ Takes a model (typically an AllenNLP
Model
, but this works for anytorch.nn.Module
) and makes a best guess about which module is the embedding layer. For typical AllenNLP models, this often is theTextFieldEmbedder
, but if you’re using a pretrained contextualizer, we really want layer 0 of that contextualizer, not the output. So there are a bunch of hacks in here for specific pretrained contextualizers.

allennlp.nn.util.
flatten_and_batch_shift_indices
(indices:torch.Tensor, sequence_length:int) → torch.Tensor[source]¶ This is a subroutine for
batched_index_select()
. The givenindices
of size(batch_size, d_1, ..., d_n)
indexes into dimension 2 of a target tensor, which has size(batch_size, sequence_length, embedding_size)
. This function returns a vector that correctly indexes into the flattened target. The sequence length of the target must be provided to compute the appropriate offsets.indices = torch.ones([2,3], dtype=torch.long) # Sequence length of the target tensor. sequence_length = 10 shifted_indices = flatten_and_batch_shift_indices(indices, sequence_length) # Indices into the second element in the batch are correctly shifted # to take into account that the target tensor will be flattened before # the indices are applied. assert shifted_indices == [1, 1, 1, 11, 11, 11]
 Parameters
 indices
torch.LongTensor
, required.  sequence_length
int
, required. The length of the sequence the indices index into. This must be the second dimension of the tensor.
 indices
 Returns
 offset_indices
torch.LongTensor
 offset_indices

allennlp.nn.util.
flattened_index_select
(target:torch.Tensor, indices:torch.LongTensor) → torch.Tensor[source]¶ The given
indices
of size(set_size, subset_size)
specifies subsets of thetarget
that each of the set_size rows should select. The target has size(batch_size, sequence_length, embedding_size)
, and the resulting selected tensor has size(batch_size, set_size, subset_size, embedding_size)
. Parameters
 target
torch.Tensor
, required. A Tensor of shape (batch_size, sequence_length, embedding_size).
 indices
torch.LongTensor
, required. A LongTensor of shape (set_size, subset_size). All indices must be < sequence_length as this tensor is an index into the sequence_length dimension of the target.
 target
 Returns
 selected
torch.Tensor
, required. A Tensor of shape (batch_size, set_size, subset_size, embedding_size).
 selected

allennlp.nn.util.
get_combined_dim
(combination:str, tensor_dims:List[int]) → int[source]¶ For use with
combine_tensors()
. This function computes the resultant dimension when callingcombine_tensors(combination, tensors)
, when the tensor dimension is known. This is necessary for knowing the sizes of weight matrices when building models that usecombine_tensors
. Parameters
 combination
str
A commaseparated list of combination pieces, like
"1,2,1*2"
, specified identically tocombination
incombine_tensors()
. tensor_dims
List[int]
A list of tensor dimensions, where each dimension is from the last axis of the tensors that will be input to
combine_tensors()
.
 combination

allennlp.nn.util.
get_device_of
(tensor:torch.Tensor) → int[source]¶ Returns the device of the tensor.

allennlp.nn.util.
get_dropout_mask
(dropout_probability:float, tensor_for_masking:torch.Tensor)[source]¶ Computes and returns an elementwise dropout mask for a given tensor, where each element in the mask is dropped out with probability dropout_probability. Note that the mask is NOT applied to the tensor  the tensor is passed to retain the correct CUDA tensor type for the mask.
 Parameters
 dropout_probabilityfloat, required.
Probability of dropping a dimension of the input.
 tensor_for_maskingtorch.Tensor, required.
 Returns
 A torch.FloatTensor consisting of the binary mask scaled by 1/ (1  dropout_probability).
 This scaling ensures expected values and variances of the output of applying this mask
and the original tensor are the same.

allennlp.nn.util.
get_final_encoder_states
(encoder_outputs:torch.Tensor, mask:torch.Tensor, bidirectional:bool=False) → torch.Tensor[source]¶ Given the output from a
Seq2SeqEncoder
, with shape(batch_size, sequence_length, encoding_dim)
, this method returns the final hidden state for each element of the batch, giving a tensor of shape(batch_size, encoding_dim)
. This is not as simple asencoder_outputs[:, 1]
, because the sequences could have different lengths. We use the mask (which has shape(batch_size, sequence_length)
) to find the final state for each batch instance.Additionally, if
bidirectional
isTrue
, we will split the final dimension of theencoder_outputs
into two and assume that the first half is for the forward direction of the encoder and the second half is for the backward direction. We will concatenate the last state for each encoder dimension, givingencoder_outputs[:, 1, :encoding_dim/2]
concatenated withencoder_outputs[:, 0, encoding_dim/2:]
.

allennlp.nn.util.
get_lengths_from_binary_sequence_mask
(mask:torch.Tensor)[source]¶ Compute sequence lengths for each batch element in a tensor using a binary mask.
 Parameters
 masktorch.Tensor, required.
A 2D binary mask of shape (batch_size, sequence_length) to calculate the perbatch sequence lengths from.
 Returns
 A torch.LongTensor of shape (batch_size,) representing the lengths
 of the sequences in the batch.

allennlp.nn.util.
get_mask_from_sequence_lengths
(sequence_lengths:torch.Tensor, max_length:int) → torch.Tensor[source]¶ Given a variable of shape
(batch_size,)
that represents the sequence lengths of each batch element, this function returns a(batch_size, max_length)
mask variable. For example, if our input was[2, 2, 3]
, with amax_length
of 4, we’d return[[1, 1, 0, 0], [1, 1, 0, 0], [1, 1, 1, 0]]
.We require
max_length
here instead of just computing it from the inputsequence_lengths
because it lets us avoid finding the max, then copying that value from the GPU to the CPU so that we can use it to construct a new tensor.

allennlp.nn.util.
get_range_vector
(size:int, device:int) → torch.Tensor[source]¶ Returns a range vector with the desired size, starting at 0. The CUDA implementation is meant to avoid copy data from CPU to GPU.

allennlp.nn.util.
get_text_field_mask
(text_field_tensors:Dict[str, torch.Tensor], num_wrapping_dims:int=0) → torch.LongTensor[source]¶ Takes the dictionary of tensors produced by a
TextField
and returns a mask with 0 where the tokens are padding, and 1 otherwise. We also handleTextFields
wrapped by an arbitrary number ofListFields
, where the number of wrappingListFields
is given bynum_wrapping_dims
.If
num_wrapping_dims == 0
, the returned mask has shape(batch_size, num_tokens)
. Ifnum_wrapping_dims > 0
then the returned mask hasnum_wrapping_dims
extra dimensions, so the shape will be(batch_size, ..., num_tokens)
.There could be several entries in the tensor dictionary with different shapes (e.g., one for word ids, one for character ids). In order to get a token mask, we use the tensor in the dictionary with the lowest number of dimensions. After subtracting
num_wrapping_dims
, if this tensor has two dimensions we assume it has shape(batch_size, ..., num_tokens)
, and use it for the mask. If instead it has three dimensions, we assume it has shape(batch_size, ..., num_tokens, num_features)
, and sum over the last dimension to produce the mask. Most frequently this will be a character id tensor, but it could also be a featurized representation of each token, etc.If the input
text_field_tensors
contains the “mask” key, this is returned instead of inferring the mask.TODO(joelgrus): can we change this? NOTE: Our functions for generating masks create torch.LongTensors, because using torch.ByteTensors makes it easy to run into overflow errors when doing mask manipulation, such as summing to get the lengths of sequences  see below. >>> mask = torch.ones([260]).byte() >>> mask.sum() # equals 260. >>> var_mask = torch.autograd.V(mask) >>> var_mask.sum() # equals 4, due to 8 bit precision  the sum overflows.

allennlp.nn.util.
has_tensor
(obj) → bool[source]¶ Given a possibly complex data structure, check if it has any torch.Tensors in it.

allennlp.nn.util.
inspect_parameters
(module:torch.nn.modules.module.Module, quiet:bool=False) → Dict[str, Any][source]¶ Inspects the model/module parameters and their tunability. The output is structured in a nested dict so that parameters in same submodules are grouped together. This can be helpful to setup module path based regex, for example in initializer. It prints it by default (optional) and returns the inspection dict. Eg. output:
{ "_text_field_embedder": { "token_embedder_tokens": { "_projection": { "bias": "tunable", "weight": "tunable" }, "weight": "frozen" } } }

allennlp.nn.util.
logsumexp
(tensor:torch.Tensor, dim:int=1, keepdim:bool=False) → torch.Tensor[source]¶ A numerically stable computation of logsumexp. This is mathematically equivalent to tensor.exp().sum(dim, keep=keepdim).log(). This function is typically used for summing log probabilities.
 Parameters
 tensortorch.FloatTensor, required.
A tensor of arbitrary size.
 dimint, optional (default = 1)
The dimension of the tensor to apply the logsumexp to.
 keepdim: bool, optional (default = False)
Whether to retain a dimension of size one at the dimension we reduce over.

allennlp.nn.util.
masked_flip
(padded_sequence:torch.Tensor, sequence_lengths:List[int]) → torch.Tensor[source]¶ Flips a padded tensor along the time dimension without affecting masked entries.
 Parameters
 padded_sequence
torch.Tensor
The tensor to flip along the time dimension. Assumed to be of dimensions (batch size, num timesteps, …)
 sequence_lengths
torch.Tensor
A list containing the lengths of each unpadded sequence in the batch.
 padded_sequence
 Returns
 A
torch.Tensor
of the same shape as padded_sequence.
 A

allennlp.nn.util.
masked_log_softmax
(vector:torch.Tensor, mask:torch.Tensor, dim:int=1) → torch.Tensor[source]¶ torch.nn.functional.log_softmax(vector)
does not work if some elements ofvector
should be masked. This performs a log_softmax on just the nonmasked portions ofvector
. PassingNone
in for the mask is also acceptable; you’ll just get a regular log_softmax.vector
can have an arbitrary number of dimensions; the only requirement is thatmask
is broadcastable tovector's
shape. Ifmask
has fewer dimensions thanvector
, we will unsqueeze on dimension 1 until they match. If you need a different unsqueezing of your mask, do it yourself before passing the mask into this function.In the case that the input vector is completely masked, the return value of this function is arbitrary, but not
nan
. You should be masking the result of whatever computation comes out of this in that case, anyway, so the specific values returned shouldn’t matter. Also, the way that we deal with this case relies on having singleprecision floats; mixing halfprecision floats with fullymasked vectors will likely give younans
.If your logits are all extremely negative (i.e., the max value in your logit vector is 50 or lower), the way we handle masking here could mess you up. But if you’ve got logit values that extreme, you’ve got bigger problems than this.

allennlp.nn.util.
masked_max
(vector:torch.Tensor, mask:torch.Tensor, dim:int, keepdim:bool=False, min_val:float=10000000.0) → torch.Tensor[source]¶ To calculate max along certain dimensions on masked values
 Parameters
 vector
torch.Tensor
The vector to calculate max, assume unmasked parts are already zeros
 mask
torch.Tensor
The mask of the vector. It must be broadcastable with vector.
 dim
int
The dimension to calculate max
 keepdim
bool
Whether to keep dimension
 min_val
float
The minimal value for paddings
 vector
 Returns
 A
torch.Tensor
of including the maximum values.
 A

allennlp.nn.util.
masked_mean
(vector:torch.Tensor, mask:torch.Tensor, dim:int, keepdim:bool=False, eps:float=1e08) → torch.Tensor[source]¶ To calculate mean along certain dimensions on masked values
 Parameters
 vector
torch.Tensor
The vector to calculate mean.
 mask
torch.Tensor
The mask of the vector. It must be broadcastable with vector.
 dim
int
The dimension to calculate mean
 keepdim
bool
Whether to keep dimension
 eps
float
A small value to avoid zero division problem.
 vector
 Returns
 A
torch.Tensor
of including the mean values.
 A

allennlp.nn.util.
masked_softmax
(vector:torch.Tensor, mask:torch.Tensor, dim:int=1, memory_efficient:bool=False, mask_fill_value:float=1e+32) → torch.Tensor[source]¶ torch.nn.functional.softmax(vector)
does not work if some elements ofvector
should be masked. This performs a softmax on just the nonmasked portions ofvector
. PassingNone
in for the mask is also acceptable; you’ll just get a regular softmax.vector
can have an arbitrary number of dimensions; the only requirement is thatmask
is broadcastable tovector's
shape. Ifmask
has fewer dimensions thanvector
, we will unsqueeze on dimension 1 until they match. If you need a different unsqueezing of your mask, do it yourself before passing the mask into this function.If
memory_efficient
is set to true, we will simply use a very large negative number for those masked positions so that the probabilities of those positions would be approximately 0. This is not accurate in math, but works for most cases and consumes less memory.In the case that the input vector is completely masked and
memory_efficient
is false, this function returns an array of0.0
. This behavior may causeNaN
if this is used as the last layer of a model that uses categorical crossentropy loss. Instead, ifmemory_efficient
is true, this function will treat every element as equal, and do softmax over equal numbers.

allennlp.nn.util.
move_to_device
(obj, cuda_device:int)[source]¶ Given a structure (possibly) containing Tensors on the CPU, move all the Tensors to the specified GPU (or do nothing, if they should be on the CPU).

allennlp.nn.util.
remove_sentence_boundaries
(tensor:torch.Tensor, mask:torch.Tensor) → Tuple[torch.Tensor, torch.Tensor][source]¶ Remove begin/end of sentence embeddings from the batch of sentences. Given a batch of sentences with size
(batch_size, timesteps, dim)
this returns a tensor of shape(batch_size, timesteps  2, dim)
after removing the beginning and end sentence markers. The sentences are assumed to be padded on the right, with the beginning of each sentence assumed to occur at index 0 (i.e.,mask[:, 0]
is assumed to be 1).Returns both the new tensor and updated mask.
This function is the inverse of
add_sentence_boundary_token_ids
. Parameters
 tensor
torch.Tensor
A tensor of shape
(batch_size, timesteps, dim)
 mask
torch.Tensor
A tensor of shape
(batch_size, timesteps)
 tensor
 Returns
 tensor_without_boundary_tokens
torch.Tensor
The tensor after removing the boundary tokens of shape
(batch_size, timesteps  2, dim)
 new_mask
torch.Tensor
The new mask for the tensor of shape
(batch_size, timesteps  2)
.
 tensor_without_boundary_tokens

allennlp.nn.util.
replace_masked_values
(tensor:torch.Tensor, mask:torch.Tensor, replace_with:float) → torch.Tensor[source]¶ Replaces all masked values in
tensor
withreplace_with
.mask
must be broadcastable to the same shape astensor
. We require thattensor.dim() == mask.dim()
, as otherwise we won’t know which dimensions of the mask to unsqueeze.This just does
tensor.masked_fill()
, except the pytorch method fills in things with a mask value of 1, where we want the opposite. You can do this in your own code withtensor.masked_fill((1  mask).to(dtype=torch.bool), replace_with)
.

allennlp.nn.util.
sequence_cross_entropy_with_logits
(logits:torch.FloatTensor, targets:torch.LongTensor, weights:torch.FloatTensor, average:str='batch', label_smoothing:float=None, gamma:float=None, alpha:Union[float, List[float], torch.FloatTensor]=None) → torch.FloatTensor[source]¶ Computes the cross entropy loss of a sequence, weighted with respect to some user provided weights. Note that the weighting here is not the same as in the
torch.nn.CrossEntropyLoss()
criterion, which is weighting classes; here we are weighting the loss contribution from particular elements in the sequence. This allows loss computations for models which use padding. Parameters
 logits
torch.FloatTensor
, required. A
torch.FloatTensor
of size (batch_size, sequence_length, num_classes) which contains the unnormalized probability for each class. targets
torch.LongTensor
, required. A
torch.LongTensor
of size (batch, sequence_length) which contains the index of the true class for each corresponding step. weights
torch.FloatTensor
, required. A
torch.FloatTensor
of size (batch, sequence_length) average: str, optional (default = “batch”)
If “batch”, average the loss across the batches. If “token”, average the loss across each item in the input. If
None
, return a vector of losses per batch element. label_smoothing
float
, optional (default = None) Whether or not to apply label smoothing to the crossentropy loss. For example, with a label smoothing value of 0.2, a 4 class classification target would look like
[0.05, 0.05, 0.85, 0.05]
if the 3rd class was the correct label. gamma
float
, optional (default = None) Focal loss[*] focusing parameter
gamma
to reduces the relative loss for wellclassified examples and put more focus on hard. The greater valuegamma
is, the more focus on hard examples. alpha
float
orList[float]
, optional (default = None) Focal loss[*] weighting factor
alpha
to balance between classes. Can be used independently withgamma
. If a singlefloat
is provided, it is assumed binary case usingalpha
and1  alpha
for positive and negative respectively. If a list offloat
is provided, with the same length as the number of classes, the weights will match the classes. [*] T. Lin, P. Goyal, R. Girshick, K. He and P. Dollár, “Focal Loss for Dense Object Detection,” 2017 IEEE International Conference on Computer Vision (ICCV), Venice, 2017, pp. 29993007.
 logits
 Returns
 A torch.FloatTensor representing the cross entropy loss.
 If
average=="batch"
oraverage=="token"
, the returned loss is a scalar.  If
average is None
, the returned loss is a vector of shape (batch_size,).

allennlp.nn.util.
sort_batch_by_length
(tensor:torch.Tensor, sequence_lengths:torch.Tensor)[source]¶ Sort a batch first tensor by some specified lengths.
 Parameters
 tensortorch.FloatTensor, required.
A batch first Pytorch tensor.
 sequence_lengthstorch.LongTensor, required.
A tensor representing the lengths of some dimension of the tensor which we want to sort by.
 Returns
 sorted_tensortorch.FloatTensor
The original tensor sorted along the batch dimension with respect to sequence_lengths.
 sorted_sequence_lengthstorch.LongTensor
The original sequence_lengths sorted by decreasing size.
 restoration_indicestorch.LongTensor
Indices into the sorted_tensor such that
sorted_tensor.index_select(0, restoration_indices) == original_tensor
 permutation_indextorch.LongTensor
The indices used to sort the tensor. This is useful if you want to sort many tensors using the same ordering.

allennlp.nn.util.
tensors_equal
(tensor1:torch.Tensor, tensor2:torch.Tensor, tolerance:float=1e12) → bool[source]¶ A check for tensor equality (by value). We make sure that the tensors have the same shape, then check all of the entries in the tensor for equality. We additionally allow the input tensors to be lists or dictionaries, where we then do the above check on every position in the list / item in the dictionary. If we find objects that aren’t tensors as we’re doing that, we just defer to their equality check.
This is kind of a catchall method that’s designed to make implementing
__eq__
methods easier, in a way that’s really only intended to be useful for tests.

allennlp.nn.util.
uncombine_initial_dims
(tensor:torch.Tensor, original_size:torch.Size) → torch.Tensor[source]¶ Given a tensor of embeddings with shape (d1 * … * dn, sequence_length, embedding_dim) and the original shape (d1, …, dn, sequence_length), return the reshaped tensor of embeddings with shape (d1, …, dn, sequence_length, embedding_dim). If original size is 1d or 2d, return it as is.

allennlp.nn.util.
viterbi_decode
(tag_sequence:torch.Tensor, transition_matrix:torch.Tensor, tag_observations:Union[List[int], NoneType]=None, allowed_start_transitions:torch.Tensor=None, allowed_end_transitions:torch.Tensor=None)[source]¶ Perform Viterbi decoding in log space over a sequence given a transition matrix specifying pairwise (transition) potentials between tags and a matrix of shape (sequence_length, num_tags) specifying unary potentials for possible tags per timestep.
 Parameters
 tag_sequencetorch.Tensor, required.
A tensor of shape (sequence_length, num_tags) representing scores for a set of tags over a given sequence.
 transition_matrixtorch.Tensor, required.
A tensor of shape (num_tags, num_tags) representing the binary potentials for transitioning between a given pair of tags.
 tag_observationsOptional[List[int]], optional, (default = None)
A list of length
sequence_length
containing the class ids of observed elements in the sequence, with unobserved elements being set to 1. Note that it is possible to provide evidence which results in degenerate labelings if the sequences of tags you provide as evidence cannot transition between each other, or those transitions are extremely unlikely. In this situation we log a warning, but the responsibility for providing selfconsistent evidence ultimately lies with the user. allowed_start_transitionstorch.Tensor, optional, (default = None)
An optional tensor of shape (num_tags,) describing which tags the START token may transition to. If provided, additional transition constraints will be used for determining the start element of the sequence.
 allowed_end_transitionstorch.Tensor, optional, (default = None)
An optional tensor of shape (num_tags,) describing which tags may transition to the end tag. If provided, additional transition constraints will be used for determining the end element of the sequence.
 Returns
 viterbi_pathList[int]
The tag indices of the maximum likelihood tag sequence.
 viterbi_scoretorch.Tensor
The score of the viterbi path.

allennlp.nn.util.
weighted_sum
(matrix:torch.Tensor, attention:torch.Tensor) → torch.Tensor[source]¶ Takes a matrix of vectors and a set of weights over the rows in the matrix (which we call an “attention” vector), and returns a weighted sum of the rows in the matrix. This is the typical computation performed after an attention mechanism.
Note that while we call this a “matrix” of vectors and an attention “vector”, we also handle higherorder tensors. We always sum over the secondtolast dimension of the “matrix”, and we assume that all dimensions in the “matrix” prior to the last dimension are matched in the “vector”. Nonmatched dimensions in the “vector” must be directly after the batch dimension.
For example, say I have a “matrix” with dimensions
(batch_size, num_queries, num_words, embedding_dim)
. The attention “vector” then must have at least those dimensions, and could have more. Both:(batch_size, num_queries, num_words)
(distribution over words for each query)(batch_size, num_documents, num_queries, num_words)
(distribution over words in a query for each document)
are valid input “vectors”, producing tensors of shape:
(batch_size, num_queries, embedding_dim)
and(batch_size, num_documents, num_queries, embedding_dim)
respectively.