[FSDP] Add some comments after reading the code. (#66956)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66956
Adds some comments I found helpful while ramping up on FSDP code.
ghstack-source-id: 141391150
Test Plan: n/a
Reviewed By: mrshenli
Differential Revision: D31780798
fbshipit-source-id: e2d38a9801b4548b202a73615774d5f0f7f5e3ed
