Add a note in DDP doc to point to ZeroRedundancyOptimizer (#53113)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/53113
Test Plan: Imported from OSS
Reviewed By: blefaudeux
Differential Revision: D26752339
Pulled By: mrshenli
fbshipit-source-id: 7a082f1007bc550eabb82b559d020bbe717fa497
