[Bugfix] Strengthen the check of X-data-parallel-rank in Hybrid LB mode

[dtcccc]

Purpose

When running vllm with hybrid LB mode, e.g.,

CUDA_VISIBLE_DEVICES=0,1 vllm serve  Qwen/Qwen2.5-7B-Instruct --port 8001 --data-parallel-size 4 --data-parallel-size-local 2  --data-parallel-hybrid-lb --enforce-eager

CUDA_VISIBLE_DEVICES=2,3 vllm serve  Qwen/Qwen2.5-7B-Instruct --port 8002 --data-parallel-size 4 --data-parallel-start-rank 2 --data-parallel-size-local 2  --data-parallel-hybrid-lb --enforce-eager

and then

port=8001 # or 8002
curl http://localhost:$port/v1/completions \
  -H "Content-Type: application/json" \
  -H "X-data-parallel-rank: 2" \
  -d '{
    "prompt": "Who are you",
    "max_tokens": 10,
    "temperature": 0
  }'

We will receive 500 Internal Server Error because index 2 is out of range about local dp size. So that the index of self.core_engines in DPLBAsyncMPClient is out of bound. Limit the X-data-parallel-rank of the request to local dp size if using hybrid LB mode to fix this issue.

After this patch, we will receive "data_parallel_rank 2 is out of range [0, 2)." as expected.

Test Plan

Test Result

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[gemini-code-assist]

Code Review

This pull request introduces a bugfix to properly validate the X-data-parallel-rank in hybrid load balancing mode. The core of the fix is in vllm/v1/engine/input_processor.py, where the validation now correctly uses the local data parallel size as the upper bound for the rank. This prevents an out-of-bounds error and ensures a proper error message is returned to the user. Additionally, a new local_engines_only property has been added to ParallelConfig, which is a good refactoring that simplifies and clarifies the code in multiple files where this condition is checked. The changes are correct and well-implemented.

[njhill]

Thanks @dtcccc, very nice changes!

[dtcccc]

@njhill Hi, would you please merge it again? The previous commit missed adaptation of a test case.