Remove reference of CUDA < 9.2 (#38977)
Summary:
Since CUDA < 9.2 is no longer supported (See https://github.com/pytorch/pytorch/pull/36848, https://github.com/pytorch/pytorch/pull/36846), this PR updates the required CUDA version in README.md to avoid confusion.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38977
Differential Revision: D21722965
Pulled By: ezyang
fbshipit-source-id: 626772f4303d023918dda34a620d95693174d97f
