Add distributed optimizer section to distributed autograd design doc. (#30068)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30068
ghstack-source-id: 94228719
Test Plan: waitforbuildbot
Differential Revision: D18556536
fbshipit-source-id: decd6927bfdd1ee3c81fef7430aa7095d7f38d33
