[CI] fix test_distributed for python 3.8+ (#36542)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36542
Python 3.8 set the default multiprocessing start mode to spawn, but we
need fork in these tests, otherwise there are some pickling issues.
Test: Ensure that these tests succeed when run with python 3.8
ghstack-source-id: 102093824
Test Plan: Ensure success with python 3.8
Differential Revision: D21007753
fbshipit-source-id: 4b39844c6ba76a53293c0dfde7c98ec5a78fe113
