add unpack_sequence and unpad_sequence functions (#66550)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/66549
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66550
Reviewed By: malfet
Differential Revision: D32299193
Pulled By: jbschlosser
fbshipit-source-id: 96c92d73d3d40b7424778b2365e0c8bb1ae56cfb
