Update references to minimum CUDA and cuDNN version (#20718)
Summary:
I didn't update the Windows references because I wasn't sure if they apply to CUDA 9. peterjc123 what should the Windows section say?
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20718
Differential Revision: D15459276
Pulled By: colesbury
fbshipit-source-id: 917e22f8ac75378d88c962c226b5a42b6799c79a
