Adds TestCase.compare_with_numpy (#39179)
Summary:
Cut from https://github.com/pytorch/pytorch/pull/38994.
This is a helper function for comparing torch and NumPy behavior. It updates the existing and increasingly popular _np_compare function and moves it to be a method on TestCase.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39179
Differential Revision: D21855082
Pulled By: mruberry
fbshipit-source-id: edca3b78ae392d32243b02bf61960898b6ba590f
