[BE] Simplify magma installation logic (#68778)
Summary:
Difference between `CUDA_VERSION` is magma package name is just a dot between major and minor
In process of refactoring, discovered that some docker images set `CUDA_VERSION` to contain minor revision, so modified pattern to strip it, i.e. `cuda-magma102` would be installed for `CUDA_VERSION=10.2.89` and `cuda-magma113` would be installed for `CUDA_VERSION=11.3.0`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68778
Reviewed By: seemethere
Differential Revision: D32605365
Pulled By: malfet
fbshipit-source-id: 43f8edeee5b55fdea6b4d9943874df8e97494ba1
