Add docs for fusion strategy (#72036)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/72036
Test Plan: Imported from OSS
Reviewed By: navahgar
Differential Revision: D33864651
Pulled By: eellison
fbshipit-source-id: b1010d97d95fd01603cc7e04a978857cbde3c4fb
(cherry picked from commit e83b008017e54c68c7be374085c3217e8e38cce5)
