Add missing `torch.distributed.ReduceOp.AVG` in type stubs (#101534)
Add missing `AVG` to `torch.distributed.ReduceOp` enum for type annotation.
Ref:
https://github.com/pytorch/pytorch/blob/88b6a4577bad670c608424018caeba8698ad5f97/torch/csrc/distributed/c10d/Types.hpp#L35-L47
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101534
Approved by: https://github.com/Skylion007
