make rpc and dist-autograd multiprocess test to use both fork and spawn (#25656)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25656
spawn multiprocessing can catch some issues that fork multiprocessing can not
catch, meanwhile fork can work properly with asan tests, but spawn
multiprocessing can not work with asan tests for some use cases right now.
so this diff adding support to launch both spawn and fork tests in
multiProcessingTestCase class, also let test_rpc and test_dist_autograd to run
both spawn and fork tests
ghstack-source-id: 91096705
Test Plan: unit tests
Reviewed By: xush6528
Differential Revision: D17086007
fbshipit-source-id: af2446e7abe948c37081cff24ed060fd87f84922
