Add `pad_sequence` as a native function (#57868)
Summary:
https://github.com/pytorch/pytorch/issues/56229
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57868
Reviewed By: mruberry
Differential Revision: D28334174
Pulled By: ngimel
fbshipit-source-id: f1647718ada596686117703b682c0af7e92e16f5
