[k8s] Run GPU labelling job only on nodes with gpus

[romilbhardwaj]

Previously python -m sky.utils.kubernetes.gpu_labeler would also try to run jobs on nodes without GPUs, which would require a manual clean up with python -m sky.utils.kubernetes.gpu_labeler --cleanup since those jobs would never complete.

This PR filters and runs gpu labelling jobs only on nodes with GPUs.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Tested on GKE and EKS cluster

[landscapepainter]

Tried out and confirmed it works smoothly. With this filtering, do we need to update L85 from tests/kubernetes/README.md as well?
Note that some jobs may be in pending state if your cluster contains CPU nodes.

[romilbhardwaj]

Thanks @landscapepainter! Merging now.