Highlight NCCL all_reduce and all_gather requirements (#18741)
Summary:
See #18689
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18741
Differential Revision: D14726874
Pulled By: mrshenli
fbshipit-source-id: a92404c653e3c62fc23fa3ccacfb3b2959b2e307
