[pytorch] Add error when PyTorch used with Python 2 (#36151)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36151
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. This commit covers `import torch` only and not C++ for now.
Test Plan: waitforsandcastle
Reviewed By: dreiss
Differential Revision: D20894381
fbshipit-source-id: a1073b7a648e07cf10cda5a99a2cf4eee5a89230
