[FSDP] Improve the documentation of state_dict (#73453)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73453
Add more documentation to state_dict and local_state_dict including
basic examplesAdd more documentation to state_dict and local_state_dict
including basic examples.
ghstack-source-id: 150107681
Test Plan: CI
Reviewed By: mrshenli
Differential Revision: D34483812
fbshipit-source-id: a5a03564c2ba75b8e627a132dab62f729a843e0d
(cherry picked from commit 4c85e83ab4421ad8bb9ab55a9850ba94289bc573)
