add type annotations to torch.nn.parallel._functions (#49687)
Summary:
Closes gh-49686
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49687
Reviewed By: ngimel
Differential Revision: D25680210
Pulled By: zou3519
fbshipit-source-id: 221f7c9a4d3a6213eac6983030b0be51ee1c5b60
