[k8s] support to use custom gpu resource name if it's not nvidia.com/gpu

[nkwangleiGIT]

We're using customized gpu resource name when using the device plugin, so we have the following GPUs in the node capability:

nvidia.com/gpu-h100
nvidia.com/gpu-h20
nvidia.com/gpu-4090

So we have to use the resource names above when launch to local K8S, such as

sky launch --image-id skypilot:20240613 --cpus 8 --memory 32 --gpus gpu-3090:2 -c my-sky-cluster --cloud kubernetes

So this PR will support to use CUSTOM_GPU_RESOURCE_NAME from environment variable to overwrite the default nvidia.com/gpu in the resources.

Tested (run the relevant ones):

• [x] Code formatting: bash format.sh
• [ ] Any manual or new tests for this PR (please specify below)
• [ ] All smoke tests: pytest tests/test_smoke.py
• [ ] Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• [ ] Backward compatibility tests: conda deactivate; bash -i tests/backward_compatibility_tests.sh

[romilbhardwaj]

Hi @nkwangleiGIT - this PR was close to being merged. Would you like to reopen it?