Rename `torch._C.Generator` to `torch.Generator` (#38773)
Summary:
Fix https://github.com/pytorch/pytorch/issues/26528
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38773
Differential Revision: D21701053
Pulled By: pbelevich
fbshipit-source-id: 57632ca9ce430ec30dc8e40739194ee2b5860f71
