Revert "Test distributed backends in parallel (#84034)"
This reverts commit 3ae5be74ac7aa4feed6ec8e7c29b280b148651a7.
Reverted https://github.com/pytorch/pytorch/pull/84034 on behalf of https://github.com/huydhn due to This somehow revives the flaky test https://github.com/pytorch/pytorch/issues/76428
