[Model] Add Stable Audio Open support for text-to-audio generation

[linyueqian]

PLEASE FILL IN THE PR DESCRIPTION HERE ENSURING ALL CHECKLIST ITEMS (AT THE BOTTOM) HAVE BEEN CONSIDERED.

Purpose

Add support for Stable Audio Open (stabilityai/stable-audio-open-1.0) for text-to-audio generation in vLLM-Omni.

Test Plan

python examples/offline_inference/text_to_audio/text_to_audio.py --model stabilityai/stable-audio-open-1.0 --prompt "The sound of a dog barking" --output dog_barking.wav

Test Result

dog_barking.wav

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft.

BEFORE SUBMITTING, PLEASE READ https://github.com/vllm-project/vllm-omni/blob/main/CONTRIBUTING.md (anything written below this line will be removed by GitHub Actions)

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[chatgpt-codex-connector]

Honor guidance_scale ≤1 from requests

guidance_scale from the request is only applied when it exceeds 1.0; otherwise the pipeline falls back to the default argument (7.0). This prevents callers from disabling classifier-free guidance or using a lower scale (e.g., requesting 0 or 1 via Omni.generate), because the model will always run CFG at scale 7 regardless of what was requested, making unconditional/low-guidance Stable Audio generation impossible.

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