Polish distributed autograd docs. (#29942)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29942
1) Added links to the design.
2) Fixed function signautres.
3) Expanded examples
ghstack-source-id: 94162372
Test Plan: waitforbuildbot
Differential Revision: D18547103
fbshipit-source-id: 067ba166c107ed14085af8ee3306d3f8a9dcebe7
