Extract common classes and functions from test_c10d to common_distributed (#23660)
Summary:
MultiProcessTestCase will be useful for both c10d and rpc tests. So, this diff extracts that class and some common decorators to a separate file.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23660
Reviewed By: pietern
Differential Revision: D16602865
Pulled By: mrshenli
fbshipit-source-id: 85ad47dfb8ba187b7debeb3edeea5df08ef690c7
