Run test_c10d.py in multi-gpu environment (#21598)
Summary:
yf225 helped me discovered that our CI does not run multi-gpu tests in `test_c10d.py`. There are quite a few multi-gpu c10d tests. This PR tries to enable those tests.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21598
Differential Revision: D15744256
Pulled By: mrshenli
fbshipit-source-id: 0a1524a862946128321f66fc8b7f331eff10e52a
