Add documentation for FeatureAlphaDropout (#36295)
Summary:
These changes add documentation for FeatureAlphaDropout, based on a need raised in an issue by SsnL (Issue https://github.com/pytorch/pytorch/issues/9886).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36295
Differential Revision: D21478591
Pulled By: zou3519
fbshipit-source-id: a73c40bf1c7e3b1f301dc3347cef7b32e9842320
