[FSDP] Backward prefetch in recursive call (#71804)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71804
Add backward prefetch arg when using auto_wrap_policy. Unittests are
updated appropriately.
ghstack-source-id: 147753214
Test Plan: CI
Reviewed By: zhaojuanmao
Differential Revision: D33782346
fbshipit-source-id: c0176b48db29c3756a8873e809610ed53480102b
(cherry picked from commit 764acb3f1c8fb9879b6c92a934df1a7d2c9e3f3d)
