Document forward AD interaction with grad mode (#72216)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/72216
Fix https://github.com/pytorch/pytorch/issues/72202
Test Plan: Imported from OSS
Reviewed By: dagitses, albanD
Differential Revision: D33991687
Pulled By: soulitzer
fbshipit-source-id: 6b074c48cb2412efdec1af1ccb050cdb2cfea4e7
(cherry picked from commit 9b87bd83b2a7155522ce9925e0d2e5d6e2f41238)
