Add documentation for hardswish (#37989)
Summary:
Fix issue https://github.com/pytorch/pytorch/issues/37431.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37989
Differential Revision: D21502182
Pulled By: zou3519
fbshipit-source-id: 245586fb555f7f1d9ec8d87269035b6fe626b47b
