[MPS] Introduce torch.mps.Event() APIs (#102121)
- Implement `MPSEventPool` to recycle events.
- Implement python bindings with `torch.mps.Event` class using the MPSEventPool backend. The current member functions of the Event class are `record()`, `wait()`, `synchronize()`, `query()`, and `elapsed_time()`.
- Add API to measure elapsed time between two event recordings.
- Added documentation for Event class to `mps.rst`.
- Added test case to `test_mps.py`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102121
Approved by: https://github.com/albanD, https://github.com/kulinseth
