type annotations for dataloader, dataset, sampler (#39392)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/38913
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39392
Reviewed By: anjali411
Differential Revision: D22102489
Pulled By: zou3519
fbshipit-source-id: acb68d9521145f0b047214d62b5bdc5a0d1b9be4
