Makes CUDA tests in test_autograd generic (#27709)
Summary:
Per title.
test_autograd.py no longer needs to import common_cuda as a result of this change.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27709
Differential Revision: D17881298
Pulled By: mruberry
fbshipit-source-id: 8b0351b65a49a072ce5ed7e7099b712847983877
