Separate magma installation for ROCm into its own file (#85567)
This aligns it with the builder repo scripts structure:
https://github.com/pytorch/builder/blob/main/common/install_rocm_magma.sh
https://github.com/pytorch/builder/blob/main/common/install_rocm.sh
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85567
Approved by: https://github.com/jeffdaily, https://github.com/huydhn
