Destroy CUDA events after profiling (#39962)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39962
Adding a simple wrapper with ref count for cuda event and
destroying cuda event after the last copy is destroyed
Test Plan: CI cuda profiler tests
Differential Revision: D22027092
Pulled By: ilia-cher
fbshipit-source-id: e0810388aa60b2291eb010896e13af1fad92e472
