Add Python declaration of torch._C and torch._C._autograd modules. (#46622)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/46622
Test Plan: Imported from OSS
Reviewed By: malfet
Differential Revision: D24761503
Pulled By: xuzhao9
fbshipit-source-id: c7ff9a9e46480a83bf6961e09972b5d20bdeb67b
