Remove unused USE_ROCM environment variable (#28641)
Summary:
All USE_ROCM logics have been moved to cmake now
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28641
Differential Revision: D18139209
Pulled By: bddppq
fbshipit-source-id: bbf0931aa6a3be963b7e0d09b6f99f088c92c94d
