FunctionEventAvg implements __iadd__ interface (#27498)
Summary:
Resolving issue https://github.com/pytorch/pytorch/issues/26433 by making FunctionEventAvg implement the `__iadd__` interface again, like it used to.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27498
Differential Revision: D17801918
Pulled By: ezyang
fbshipit-source-id: 0597059c903ac168ed64a05ac1decff3ffd14f06
