Summary:
This will help releasing models that are using Caffe2 but have their own operator implementations and extensions. More detailed docs to arrive later. Let's see what contbuild says.
Closes https://github.com/caffe2/caffe2/pull/1378
Differential Revision: D6155045
Pulled By: Yangqing
fbshipit-source-id: 657a4c8de2f8e095bad5ed5db5b3e476b2a877e1
