Summary:
The pytorch interop code was in a test until now. However, it is better if people can rely on it to be updated when the API is updated. Therefore, we move it into contrib.
Also added a README.md
Reviewed By: wickedfoo
Differential Revision: D23392962
fbshipit-source-id: 9b7c0e388a7ea3c0b73dc0018322138f49191673