BF16 allreduce hook (#63260)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63260
Add BF16 all-reduce communication hook. Skip if CUDA version < 11 or NCCL version < 2.9.7.
Reviewed By: SciPioneer
Differential Revision: D30238317
fbshipit-source-id: bad35bf7d43f10f1c40997a282b831b61ef592bb
