Move all Stream and Event Python implementation to C++ (#15937)
Summary:
1. Added `torch/csrc/cuda/Event.h` and `torch/csrc/cuda/Event.cpp` to bind Python Event class to C++ implementation.
2. Move all CUDA runtime invocations from `torch/cuda/streams.py` to C++
3. Added tests to cover Stream and Event APIs. ~(event IPC handle tests is introduced in #15974)~
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15937
Differential Revision: D13649001
Pulled By: mrshenli
fbshipit-source-id: 84ca58f35f6ba679a4ba33150ceba678d760d240
