GitLab

* [minor] use dist.group.WORLD for default process group

- this is slightly more efficient than the previous commit
  for get_process_group_cached.

* fix

* better fix

* fixed for pytorch 1.6 and 1.7

* Update fairscale/utils/parallel.py
Co-authored-by: Min Xu <min.xu@acm.org>
Co-authored-by: Min Xu <min.xu.public@gmail.com>