[Bugfix] Restore multimodal support on the plain "vllm" throughput backend

[mganczarenko]

Purpose

vllm bench throughput --backend vllm crashes on any multimodal dataset (e.g. ASR/Whisper) with:

ValueError: Multi-modal content is not supported on backend 'vllm'; use one of [].

Regression from #50981. get_requests() computed the multimodal-backend allowlist from the backend already selected instead of from what the backend can actually handle:

mm_backends = ("vllm-chat",) if args.backend == "vllm-chat" else ()

This is circular — it only lets the already-selected backend through the gate, and only if it's literally "vllm-chat". So --backend vllm always computes mm_backends=() and self-rejects, even though _run_vllm_requests (the function that executes --backend vllm requests) already
forwards multi_modal_data straight into LLM.generate — it was always multimodal-capable, same as vllm-chat. Only hf (asserts prompts must be str) and mii (text-only) genuinely can't carry multimodal content, so the allowlist should be static rather than derived from the chosen
backend:

  mm_backends = ("vllm", "vllm-chat")

Test Plan

• Added test_get_requests_allows_multimodal_on_plain_vllm_backend to tests/benchmarks/test_throughput_cli.py, mirroring the existing test_get_requests_resolves_mmvu pattern: stubs ASRDataset (network-free), parses --backend vllm --dataset-name hf --dataset-path
  openslr/librispeech_asr, asserts get_requests() resolves without raising.
• Ran the exact vllm bench throughput command from the bug report end-to-end on real hardware (Intel XPU B70) before and after the fix.
• ruff check / ruff format on both changed files.

Test Result

• New test: fails with the reported ValueError against pre-fix code, passes against post-fix code.
• python -m pytest tests/benchmarks/test_throughput_cli.py -k "allows_multimodal_on_plain_vllm_backend or resolves_mmvu" → 2 passed.
• End-to-end on B70: pre-fix reproduces the ValueError verbatim; post-fix completes cleanly (Throughput: 20.96 requests/s, 5386.74 total tokens/s).
• ruff check: all checks passed. ruff format --diff: no changes.

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

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[DarkLight1337]

cc @JaredforReal