[BE] Simplify `Source.is_nn_module`; add some types (#95292)
I am still reading Dynamo source code...
This is an easy PR to simplify `Source.is_nn_module()` to reuse `GuardSource.is_nn_module()` instead of having the `in (...)` check implemented twice. While simplifying that, I thought I might as well add some type annotations for `Source` methods.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95292
Approved by: https://github.com/ezyang
