[Misc] Point the unquantized-MoE backend error at the speculative-config fix

[spped2000]

Problem

Serving a quantized MoE target with a speculative draft model whose MoE layers are unquantized applies the target's --moe-backend to the drafter too, which fails:

ValueError: moe_backend='flashinfer_b12x' is not supported for unquantized MoE.
            Expected one of ['triton', 'flashinfer_trtllm', 'flashinfer_cutlass', 'aiter'].

The message is accurate but incomplete: it reads as this combination is unsupported, when in fact vLLM already supports it — SpeculativeConfig.moe_backend (vllm/config/speculative.py) sets the drafter's backend independently, and its own docstring names this exact case ("quantized generator with unquantized drafter").

I spent a while concluding the configuration was impossible before finding that field. One sentence in the error would have prevented it.

Change

moe_backend='flashinfer_b12x' is not supported for unquantized MoE.
Expected one of ['triton', 'flashinfer_trtllm', 'flashinfer_cutlass', 'aiter'].
If 'flashinfer_b12x' is meant for a quantized model and this is an unquantized
draft model, set `moe_backend` inside `--speculative-config` instead, which
applies to the drafter only.

Message-only; no behaviour change.

Encountered on vLLM 0.25.1, NVIDIA GB10 (SM121), verified the code path and the SpeculativeConfig.moe_backend field are unchanged on current main.

---

Disclosure: prepared with assistance from Claude (Anthropic).

[claude]

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