pytorch

305515de - [v1.7 patch] Disallow creation of ProcessGroupNCCL without GPUs. (#45642) (#46073)

[v1.7 patch] Disallow creation of ProcessGroupNCCL without GPUs. (#45642) (#46073) Summary: Note: This PR has been merged into master at b5a2f04089 after the 1.7 branch cut (see original PR: #45642). This PR is to merge it into the 1.7 branch. ---- Original Commit Description Follows --- Pull Request resolved: https://github.com/pytorch/pytorch/pull/45642 Prior to https://github.com/pytorch/pytorch/pull/45181, initializing a NCCL process group would work even if no GPUs were present. Although, now since init_process_group calls `barrier()` this would fail. In general the problem was that we could initialize ProcessGroupNCCL without GPUs and then if we called a method like `barrier()` the process would crash since we do % numGPUs resulting in division by zero. ghstack-source-id: 113490343 Test Plan: waitforbuildbot Reviewed By: osalpekar Differential Revision: D24038839 fbshipit-source-id: a1f1db52cabcfb83e06c1a11ae9744afbf03f8dc Co-authored-by: Pritam Damania <pritam.damania@fb.com>