Bump up NCCL to 2.7.6 (#42645)
Summary:
Because 2.7.3 has some bug on GA100 which is fixed in 2.7.6
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42645
Reviewed By: malfet
Differential Revision: D22977280
Pulled By: mrshenli
fbshipit-source-id: 74779eff90d7d660a988ff33659f3a2237ca7e29
