[pytorch] Add C++ error when PyTorch used with Python 2
Summary: Python 2 has reached end-of-life and is no longer supported by PyTorch. To avoid confusing behavior when trying to use PyTorch with Python 2, detect this case early and fail with a clear message in C++.
Test Plan: waitforsandcastle
Reviewed By: orionr
Differential Revision: D21043062
fbshipit-source-id: ab448d2888f5048a0180598b882adfc67e31d851
