Add event and event_counter columns to caffe2_usage_tracer table (#21739)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21739
Added event and event_counter columns for PyTorch/Caffe2 API usage metrics
Reviewed By: dzhulgakov
Differential Revision: D15119119
fbshipit-source-id: a71010bd659109a8e4f3a8bad84b22c1d15dc528
