graph mode: more docs for insert observers pass (#39739)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39739
Adding more docs and examples to make code reasing easier for newcomers.
Test Plan:
CI, no logic changes
Imported from OSS
Differential Revision: D21975878
fbshipit-source-id: 464858c0490cfbdec165a5ddf3817ca4878abb09
