[dist_autograd] expose distributed backward C++ API (#38656)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/38656
Test Plan: Imported from OSS
Differential Revision: D21940441
Pulled By: wanchaol
fbshipit-source-id: e9d35201825912f5e7d7e1d0a71586abe5a6f71c
