Update NCCL from 2.4.8 to 2.7.3 (#41608)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/41608
Reviewed By: mrshenli, ngimel
Differential Revision: D22604953
Pulled By: malfet
fbshipit-source-id: 28151e2d5b6ea360b79896cb79c761756687d121
