add type annotations to multiprocessing module (#47756)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/47757
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47756
Reviewed By: malfet
Differential Revision: D24970773
Pulled By: ezyang
fbshipit-source-id: b0b9edb9cc1057829c6320e78174c6d5f7a77477
