[Profiler] Abstract interface for Python tracer
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77699
The current machinery to connect libtorch to libtorch_python for profiling is... meh. Adequite for separate components that mostly just need to send a trigger, but not really clean. This PR makes an abstract interface class that the python tracer subclasses so the profiler can actually get at the tracer singleton, albeit through a restricted interface. This will help fold Python tracing into the new unified event structure.
Differential Revision: [D36325739](https://our.internmc.facebook.com/intern/diff/D36325739/)
Approved by: https://github.com/aaronenyeshi
