`MultiProcessTestCase` to use instance rather than class method wrappers (#36826)
Summary:
This makes its wrappers stackable with `common_utils.TestCase` ones
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36826
Test Plan: CI
Differential Revision: D21178217
Pulled By: mrshenli
fbshipit-source-id: f80dd4aa175e20bd338b38b2c42c3118258f45dc
