Add LSTM to standard library (#15744)

Add LSTM to standard library (#15744)

Summary:
**WIP**

Attempt 2 at #14831

This adds `nn.LSTM` to the jit standard library. Necessary changes to the module itself are detailed in comments. The main limitation is the lack of a true `PackedSequence`, instead this PR uses an ordinary `tuple` to stand in for `PackedSequence`.

Most of the new code in `rnn.py` is copied to `nn.LSTM` from `nn.RNNBase` to specialize it for LSTM since `hx` is a `Tuple[Tensor, Tensor]` (rather than just a `Tensor` as in the other RNN modules) for LSTM.

As a hack it adds an internal annotation `@_parameter_list` to mark that a function returns all the parameters of a module. The weights for `RNN` modules are passed to the corresponding op as a `List[Tensor]`. In Python this has to be gathered dynamically since Parameters could be moved from CPU to GPU or be deleted and replaced (i.e. if someone calls `weight_norm` on their module, #15766), but in the JIT parameter lists are immutable, hence a builtin to handle this differently in Python/JIT.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15744

Differential Revision: D14173198

Pulled By: driazati

fbshipit-source-id: 4ee8113159b3a8f29a9f56fe661cfbb6b30dffcd