C++/Python API Parity: add pad_sequence (#32387)
Summary:
- add `pad_sequence` and tests
- related issue https://github.com/pytorch/pytorch/issues/25883
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32387
Differential Revision: D20025421
Pulled By: yf225
fbshipit-source-id: caa9ae2114bece8db387a3a1610f24a3e06b1324
