🚀[Feature Request]: Add support for CPU-only partition

### A clear and concise description of what you want to happen.

Some of the AI clusters may only have CPUs to accommodate non-GPU computing needs. ClusterScope needs to support cpu-only clusters or a partition within a Slurm cluster that only has CPUs.

This should translate into not-erroring out when we can't see any GPUs in a cluster.

### A clear and concise description of any alternative solutions or features you've considered, if any.

_No response_

### Additional context

_No response_

Start by tracing how ClusterScope discovers GPUs and handles Slurm cluster or partition information. Run the existing test suite, then verify that a CPU-only cluster or CPU-only partition no longer causes an error and that normal GPU-backed clusters still behave as before.

Written by the indexing model from the issue text.