allow set_multithreading_enabled to act as function and context manager (#105291)
Fixes #104985
Implemented `set_multithreading_enabled` C++ function to directly alter state rather than using `MultithreadingEnabled` class, which was automatically resetting the state when the object was destroyed. This behavior more closely aligns with set_grad_enabled which does work as expected. This allows us to change python class `set_multithreading_enabled` to act as both a function and context manager.
I also added a getter: `torch._C.is_multithreading_enabled`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105291
Approved by: https://github.com/albanD
