Remove obsolete warning message from DDP (#40190)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40190
Fixed by #36503
Test Plan: Imported from OSS
Differential Revision: D22101516
Pulled By: mrshenli
fbshipit-source-id: 9abd6dce602530c11b7fe623ac0f4d556dccc961
