[bugfix] fix ray start failed: local_world_size cannot little than visible device count error

[leo-pony]

What this PR does / why we need it?

Fix the ray start failed bug: local_world_size cannot little than visible device count error
detail see issue #4456.

This fix code is copied from vllm fixing modify, PR: #28873

Does this PR introduce any user-facing change?

How was this patch tested?

• vLLM version: v0.11.2

[gemini-code-assist]

Code Review

This pull request fixes a bug where ray start would fail due to an incorrect assertion on local_world_size. The change correctly adds a condition to skip this assertion when using ray or external_launcher as the distributed backend. The fix seems correct and addresses the issue described. I have one suggestion to improve code consistency and readability.

[github-actions]

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[leo-pony]

@wangxiyuan It's ready for review. Could your help review?
Test case:
I test with 2 * atlas A3 nodes, with pp2 * tp16, successfully starting and running inference:
Ray start successfully, and qwen3-235B-30B successully running:


The inference request successfully returned: