[C++ API] Add PackedSequence / pack_padded_sequence / pad_packed_sequence / pack_sequence (#33652)
Summary:
Most of the function implementation and test code are translated from the Python version.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33652
Differential Revision: D20052211
Pulled By: yf225
fbshipit-source-id: ce6767db54364f91ef4f06674239a12278c2752a
