Document unavailable reduction ops with NCCL backend (#42822)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42822
These ops arent supported with NCCL backend and used to silently error.
We disabled them as part of addressing https://github.com/pytorch/pytorch/issues/41362, so
document that here.
ghstack-source-id: 109957761
Test Plan: CI
Reviewed By: mrshenli
Differential Revision: D23023046
fbshipit-source-id: 45d69028012e0b6590c827d54b35c66cd17e7270
