add type annotations to torch._utils (#49705)
Summary:
closes gh-49704
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49705
Reviewed By: mruberry
Differential Revision: D25725352
Pulled By: malfet
fbshipit-source-id: 05a7041c9caffde4a5c1eb8af0d13697075103af
