[refactor] Flatten Ming-flash-omni-2.0 imagegen args

[yuanheng-zhao]

PLEASE FILL IN THE PR DESCRIPTION HERE.

Purpose

1. Clean up the nested args resolution in Ming-flash-omni-2.0 image-gen pipeline.
2. Prerequisite for [RFC sub-task] Migrate ming_flash_omni example to standard task example + model_extras #4549

Test Plan

Launch the server

vllm serve Jonathan1909/Ming-flash-omni-2.0 --omni \
    --deploy-config vllm_omni/deploy/ming_flash_omni_image.yaml \
    --port 8091

Send requests with and without byte5_text

curl http://127.0.0.1:8091/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "Jonathan1909/Ming-flash-omni-2.0",
    "modalities": ["image"],
    "sampling_params_list": [
      {
        "temperature": 0.4,
        "top_p": 0.9,
        "top_k": 1,
        "max_tokens": 1,
        "seed": 42,
        "extra_args": {
          "negative_prompt": "ugly, blurry, distorted"
        }
      },
      {
        "seed": 42,
        "extra_args": {
          "steps": 6,
          "cfg": 1.5,
          "height": 512,
          "width": 512,
          "seed": 123,
          "byte5_text": ["理解与生成统一"]
        }
      }
    ],
    "messages": [
      {
        "role": "user",
        "content": "Draw a poster."
      }
    ]
  }' \
  | jq -r '.choices[0].message.content[0].image_url.url | split(",")[1]' \
  | base64 -d > ming_imagegen_knobs.png

vLLM Version: 0.23.0

vLLM-Omni Commit: 1292c47

Test Result

without byte5_text	with byte5_text

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[hsliuustc0106]

2 blocking issues above

[chatgpt-codex-connector]

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[yuanheng-zhao]

PTAL @ZhengWG , @princepride

[princepride]

vllm_omni/diffusion/models/ming_flash_omni/pipeline_ming_imagegen.py#L239-L241

Seems byte5_text can be string, but you did't consider this type, right?

[yuanheng-zhao]

Good catch. Revised as allowing str as well

[princepride]

LGTM, just left some comments.

[ZhengWG]

LGTM， this makes the byte5_text flow cleaner, Thx!

lgtm