[FX] Add a bunch of docstrings (#47719)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/47719
Test Plan: Imported from OSS
Reviewed By: zdevito
Differential Revision: D24875400
Pulled By: jamesr66a
fbshipit-source-id: a1dd43d2eee914a441eff43c4f2efe61a399e8a5
