Improve docs around Math/DefaultBackend & add PythonDispatcher class. (#50854)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/50854
Test Plan: Imported from OSS
Reviewed By: bhosmer
Differential Revision: D26008542
Pulled By: ailzhang
fbshipit-source-id: e9c0aa97ac2537ff612f5faf348fcb613da09479
