functional autograd api (#34066)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34066
Basic implementation of https://github.com/pytorch/pytorch/issues/30632
Test Plan: Imported from OSS
Differential Revision: D20260307
Pulled By: albanD
fbshipit-source-id: 7db5c2411ddc3e954ff8fbbe93eb3b96a2bcfb2f
