[RELAND] Enable ncclAvg for reductions (#62835)
Summary:
Resubmit of https://github.com/pytorch/pytorch/pull/62303.
Reverts the revert, and restores some diffs that were mysteriously missing from the reverted revert. I think some of the diffs I pushed to the original PR raced with its import or landing, such that the original PR's merge didn't pick up all the diffs I wanted. I don't know enough about the landing process to do more than speculate wildly, but hopefully this resubmit sorts things out.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62835
Reviewed By: zhouzhuojie, seemethere, janeyx99, heitorschueroff
Differential Revision: D30999982
Pulled By: malfet
fbshipit-source-id: 1f70ab4055208f1c6a80c9fc9fbe292ce68ecaa9
