Back out "Make grad point to bucket buffer in DDP to save memory usage" (#43557)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43557
backout the diff that caused some errors in pytext distributed training
Test Plan: Tested by rayhou who verified reverting the diff works
Differential Revision: D23320238
fbshipit-source-id: caa0fe74404059e336cd95fdb41373f58ecf486e
