Stop using deprecated torch.distributed.reduce_op
Summary:
Using `torch.distributed.reduce_op` raises a deprecation warning in saying that we should use `torch.distributed.ReduceOp`. I don't know exactly where this warning is coming from in my code, but I figured that we might as well just upgrade everyone at once.
This was done using BigGrep, although I avoided touching `fbcode/pytorch/apex/third-party` and `fbcode/caffe2` since that has a different merge procedure on Github.
Reviewed By: joseph-zhong
Differential Revision: D60989744
fbshipit-source-id: 2a606fca4bb2c467032e694154f4ee5107e91676
