Add comments to explain how MultiProcessTestCase works (#37179)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/37179
Test Plan: Imported from OSS
Differential Revision: D21211196
Pulled By: mrshenli
fbshipit-source-id: 86dc8183b4def7e95a236f6c5c73ef67466f4ddb
