[ROCm] remove use of HIP_PLATFORM (#47241)
Summary:
Fixes deprecated use of the HIP_PLATFORM env var. This env var is no longer needed to be set explicitly. Instead, HIP_PLATFORM is automatically detected by hipcc.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47241
Reviewed By: mruberry
Differential Revision: D24699982
Pulled By: ngimel
fbshipit-source-id: 9cd2f32e7c0c8d662832b0cbbc2988835a45961a
