Allow existing "Python RAII guards" to be used as context managers (#102579)
This PR adds a `py_context_manager_DEPRECATED` that converts a C++ RAII
guard to an object that may be either used as Python context manager or
as a "Python RAII guard".
We don't convert all of them to Python context manager only due to BC
reasons; people in OSS and internally actually rely on these APIs and I
don't want to break them. We are justified in breaking BC if we wanted
to, but it seemed like too much work for not a lot of gain.
The API is postfixed with "DEPRECATED" to indicate that people should
really use `py_context_manager` (converts C++ RAII guard to Python
context manager) instead.
Test Plan:
- this PR converts all PyTorch usages of _AutoDispatchBelowAutograd to
context manager. I can do the rest in follow-ups.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102579
Approved by: https://github.com/bdhirsh, https://github.com/albanD
