Disallow creation of ProcessGroupNCCL without GPUs. (#45642)
Summary:
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
