PytorchSeq2VecWrapper(self, module:torch.nn.modules.rnn.RNNBase) -> None

Pytorch's RNNs have two outputs: the hidden state for every time step, and the hidden state at the last time step for every layer. We just want the second one as a single output. This wrapper pulls out that output, and adds a :func:get_output_dim method, which is useful if you want to, e.g., define a linear + softmax layer on top of this to get some distribution over a set of labels. The linear layer needs to know its input dimension before it is called, and you can get that from get_output_dim.

Also, there are lots of ways you could imagine going from an RNN hidden state at every timestep to a single vector - you could take the last vector at all layers in the stack, do some kind of pooling, take the last vector of the top layer in a stack, or many other options. We just take the final hidden state vector, or in the case of a bidirectional RNN cell, we concatenate the forward and backward final states together. TODO(mattg): allow for other ways of wrapping RNNs.

In order to be wrapped with this wrapper, a class must have the following members:

- `self.input_size: int`
- `self.hidden_size: int`
- `def forward(inputs: PackedSequence, hidden_state: torch.tensor) ->
  Tuple[PackedSequence, torch.Tensor]`.
- `self.bidirectional: bool` (optional)

This is what pytorch's RNN's look like - just make sure your class looks like those, and it should work.

Note that we require you to pass sequence lengths when you call this module, to avoid subtle bugs around masking. If you already have a PackedSequence you can pass None as the second parameter.