torch/monitor: add C++ events and handlers (#68783)
Summary:
This adds a C++ event handler corresponding to the Python one mentioned in the RFC.
This changes the counters a bit to all be push driven instead of being polled. The two window types are "fixed count" and "interval". One is based off the number of logged events and the other is based off of time windows. There's currently no active ticker for interval so it needs a regular stream of events to ensure events are produced. A follow up diff can add support for things like HHWheel / simple ticker.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68783
Test Plan: buck test //caffe2/test/cpp/monitor:monitor
Reviewed By: kiukchung
Differential Revision: D32606547
fbshipit-source-id: a00d0364092d7d8a98e0b18e503c0ca8ede2bead
