a (major,minor) pair that indicates the minimum
CUDA compute capability required, or None if no requirement.
Note that the keyword arg name "cuda_only" is misleading (since routine will
return true when a GPU device is available irrespective of whether TF was
built with CUDA support or ROCm support. However no changes here because
++ Changing the name "cuda_only" to something more generic would break
backward compatibility
++ Adding an equivalent "rocm_only" would require the implementation check
the build type. This in turn would require doing the same for CUDA and thus
potentially break backward compatibility
++ Adding a new "cuda_or_rocm_only" would not break backward compatibility,
but would require most (if not all) callers to update the call to use
"cuda_or_rocm_only" instead of "cuda_only"
Returns
True if a GPU device of the requested kind is available.