More type stubs (#18511)

More type stubs (#18511)

Summary:
Added stubs for:

* The `device` module
* The `cuda` module
* Parts of the `optim` module
* Began adding stubs for the `autograd` module. I'll annotate more later but `no_grad` and friends are probably the most used exports from it so it seemed like a good place to start.

This would close #16996, although comments on that issue reference other missing stubs so maybe it's worth keeping open as an umbrella issue.

The big remaining missing package is `nn`.

Also added a `py.typed` file so mypy will pick up on the type stubs. That closes #17639.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18511

Differential Revision: D14715053

Pulled By: ezyang

fbshipit-source-id: 9e4882ac997063650e6ce47604b3eaf1232c61c9