[WIP] make test_distributed gloo test use MultiProcessTestCase (#36970)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36970
We would like to move all distributed testing to use the existing
multiprocessing tooling defined in common_distributed.py. With this change, we
make `TestDistBackend` inherit from `MultiProcessTestCase` and enable fork mode
multiprocessing. In the next step, we can enable spawn mode for these tests
which will give us TSAN coverage.
ghstack-source-id: 102553801
Test Plan: Unittests
Differential Revision: D21146947
fbshipit-source-id: 608fa2cb93e88f8de6a5ac87c523e2c4e4fede1b
