[C++ API] RNNCell / LSTMCell / GRUCell layers (#34400)
Summary:
This PR adds `RNNCell` / `LSTMCell` / `GRUCell` layers to the C++ frontend, with implementations exactly matching the Python API equivalent.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34400
Differential Revision: D20316859
Pulled By: yf225
fbshipit-source-id: bb7cee092622334043c0d0fd0fcb4e75e707699c
