GLOO process group GPU alltoall (#41690)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41690
Gloo alltoall for GPU
Test Plan: buck test mode/dev-nosan caffe2/torch/lib/c10d:ProcessGroupGlooTest
Reviewed By: osalpekar
Differential Revision: D22631554
fbshipit-source-id: 4b126d9d991a118f3925c005427f399fc60f92f7
