[Distributed] Simplify MNNVL capability check

[GirasoleY]

Summary

Follow-up cleanup for vllm-project#53253:

• use the existing platform capability helper for the SM100 gate;
• query the communicator's actual CUDA device rather than the default device;
• preserve the Blackwell, local-multicast, and group-consensus behavior;
• update the focused test to cover a non-default CUDA device index.

This targets vllm-project#53253's head branch directly. It is not a competing upstream PR or an alternative implementation of the bug fix.

Validation

• VLLM_TARGET_DEVICE=cpu .venv/bin/python -m pytest tests/distributed/test_custom_all_reduce.py -q -k 'cross_node_mnnvl_gate or local_multicast_support': 6 passed, 4 deselected.
• Ruff check and format: passed.
• Repository pre-commit hooks, including mypy and commit sign-off: passed.
• git diff --check: passed.

No model evaluation is needed because this only refines capability detection and its unit test; it does not affect model output or accuracy.

AI assistance

Codex assisted with review, implementation, and validation. The submitting human is responsible for reviewing and understanding every changed line before merge.

[github-actions]

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