Adding docstring to the observers (#27791)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27791
This is the first part of the change. The next ones will amend more :)
Test Plan: Imported from OSS
Differential Revision: D17889913
Pulled By: z-a-f
fbshipit-source-id: ff74007903dd789d4c68684e83b50c0c86a25149
