[C++ API] RNN / GRU / LSTM layer refactoring (#34322)
Summary:
This PR refactors RNN / GRU / LSTM layers in C++ API to exactly match the implementation in Python API.
**BC-breaking changes:**
- Instead of returning `RNNOutput`, RNN / GRU forward method now returns `std::tuple<Tensor, Tensor>`, and LSTM forward method now returns `std::tuple<Tensor, std::tuple<Tensor, Tensor>>`, matching Python API.
- RNN / LSTM / GRU forward method now accepts the same inputs (input tensor and optionally hidden state), matching Python API.
- RNN / LSTM / GRU layers now have `forward_with_packed_input` method which accepts `PackedSequence` as input and optionally hidden state, matching the `forward(PackedSequence, ...)` variant in Python API.
- RNN / LSTM / GRU layers no longer have these fields: `w_ih` / `w_hh` / `b_ih` / `b_hh`. Instead, to access the weights and biases of the gates, users should do e.g. `rnn->named_parameters()["weight_ih_l0"]`, which mirrors the Python API `rnn.weight_ih_l0`.
- In `RNNOptions`
- `tanh()` / `relu()` / `activation` are removed. Instead, `nonlinearity` is added which takes either `torch::kTanh` or `torch::kReLU`
- `layers` -> `num_layers`
- `with_bias` -> `bias`
- In `LSTMOptions`
- `layers` -> `num_layers`
- `with_bias` -> `bias`
- In `GRUOptions`
- `layers` -> `num_layers`
- `with_bias` -> `bias`
The majority of the changes in this PR focused on refactoring the implementations in `torch/csrc/api/src/nn/modules/rnn.cpp` to match the Python API. RNN tests are then changed to reflected the revised API design.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34322
Differential Revision: D20458302
Pulled By: yf225
fbshipit-source-id: ffff2ae1ddb1c742c966956f6ad4d7fba03dc54d
