Revert D29342234: [pytorch][PR] [CUDA graphs][BC-breaking] Removes post-backward syncs on default stream
Test Plan: revert-hammer
Differential Revision:
D29342234 (https://github.com/pytorch/pytorch/commit/675cea1adb852db5c60ee3250e5b3ae3a5da7f11)
Original commit changeset: 98e6be7fdd85
fbshipit-source-id: 84022973248b2254210eee57402df2c4f4bc43c6
