feat: add explicit GPU identity detector

[rhewett-nv]

What does this PR do?

Adds a best-effort NVML detector for worker-local nv.gpu.* resource attributes.

Issues

N/A

Usage

from nemo.lens.resources import detect_gpu

gpu_attributes = detect_gpu(local_rank=local_rank)

# Before your PR is "Ready for review"

Pre checks:

- [x] I read the Contributor guidelines (/NVIDIA-NeMo/lens/blob/main/CONTRIBUTING.md).
- [x] All commits are signed off (git commit -s) per the DCO.
- [x] I added or updated tests under tests/ for any behavior change.
- [x] I ran pytest and pre-commit run --all-files locally; both pass.
- [x] I updated docs/ and README.md where behavior, configuration, or public API changed.
- [x] If this changes a lens public API symbol, I updated the cross-repo _fallbacks.py files in Megatron-LM / RL / Gym.

# Additional information

- GPU detection returns an empty mapping when NVML or a usable device is unavailable.

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[ahmadki]

/ok to test 4f5465d

[ahmadki]

/ok to test bbc009e

The merge-base changed after approval.