Add option to run python unittests in parallel (#37180)
Summary:
So far results looks quite promising: test_nn is purely sequential tests and can be accelerated 3x
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37180
Differential Revision: D21437871
Pulled By: malfet
fbshipit-source-id: 8679a8af355f839f2c9dae3bf36d2e102af05425
