autograd/profiler: support merging FunctionEventAvg (#30677)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30677
Currently you can only add FunctionEvents to FunctionEventAvg. This makes it so you can add multiple FunctionEventAvg objects together. This is useful for merging multiple profiles together such as when dealing with distributed training.
Test Plan:
added unit test
buck test //caffe2/test:autograd -- test_profiler
Reviewed By: bddppq
Differential Revision: D18785578
fbshipit-source-id: 567a441dec885db7b0bd8f6e0ac9a60b18092278
