[DDP Comm Hook] Add bf16 gradient compression to ddp_comm_hooks.rst (#64346)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64346
as title
ghstack-source-id: 137170288
Test Plan: N/A
Reviewed By: rohan-varma
Differential Revision: D30693513
fbshipit-source-id: 8c64b8404ff3b0322e1bbbd93f6ef051ea91307d
