Move torch/autograd/grad_mode.pyi stubs inline (#43415)
Summary:
- Add `torch._C` bindings from `torch/csrc/autograd/init.cpp`
- Renamed `torch._C.set_grad_enabled` to `torch._C._set_grad_enabled`
so it doesn't conflict with torch.set_grad_enabled anymore
This is a continuation of gh-38201. All I did was resolve merge conflicts and finish the annotation of `_DecoratorContextManager.__call__` that ezyang started in the first commit.
~Reverts commit b5cd3a80bbc, which was only motivated by not having `typing_extensions` available.~ (JIT can't be made to understand `Literal[False]`, so keep as is).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43415
Reviewed By: ngimel
Differential Revision: D23301168
Pulled By: malfet
fbshipit-source-id: cb5290f2e556b4036592655b9fe54564cbb036f6
