Add online serving to Stable Audio Diffusion and introduce v1/audio/generate endpoint

docs/serving/audio_generate_api.md

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+# Audio Generate API
+
+vLLM-Omni provides an API for text-to-audio generation using diffusion-based models such as Stable Audio.
+
+Unlike the [Speech API](speech_api.md) which targets text-to-speech synthesis, the Audio Generate API is designed for general-purpose audio generation from text descriptions (sound effects, music, ambient soundscapes, etc.).
+
+Each server instance runs a single model (specified at startup via `vllm-omni serve <model> --omni`).
+
+## Quick Start
+
+### Start the Server
+
+```bash
+vllm-omni serve stabilityai/stable-audio-open-1.0 \
+    --host 0.0.0.0 \
+    --port 8000 \
+    --gpu-memory-utilization 0.9 \
+    --trust-remote-code \
+    --enforce-eager \
+    --omni
+```
+
+### Generate Audio
+
+**Using curl:**
+
+```bash
+curl -X POST http://localhost:8000/v1/audio/generate \
+    -H "Content-Type: application/json" \
+    -d '{
+        "input": "The sound of a cat purring",
+        "audio_length": 10.0
+    }' --output cat.wav
+```
+
+**Using Python:**
+
+```python
+import httpx
+
+response = httpx.post(
+    "http://localhost:8000/v1/audio/generate",
+    json={
+        "input": "The sound of a cat purring",
+        "audio_length": 10.0,
+    },
+    timeout=300.0,
+)
+
+with open("cat.wav", "wb") as f:
+    f.write(response.content)
+```
+
+## API Reference
+
+### Endpoint
+
+```
+POST /v1/audio/generate
+Content-Type: application/json
+```
+
+### Request Parameters
+
+| Parameter | Type | Default | Description |
+|-----------|------|---------|-------------|
+| `input` | string | **required** | Text prompt describing the audio to generate |
+| `model` | string | server's model | Model to use (optional, should match server if specified) |
+| `response_format` | string | "wav" | Audio format: wav, mp3, flac, pcm, aac, opus |
+| `speed` | float | 1.0 | Playback speed (0.25 - 4.0) |
+
+#### Diffusion Parameters
+
+| Parameter | Type | Default | Description |
+|-----------|------|---------|-------------|
+| `audio_length` | float | null | Audio duration in seconds (default value is the max ~47s for `stable-audio-open-1.0`) |
+| `audio_start` | float | 0.0 | Audio start time in seconds |
+| `negative_prompt` | string | null | Text describing what to avoid in generation |
+| `guidance_scale` | float | model default | Classifier-free guidance scale (higher = more adherence to prompt) |
+| `num_inference_steps` | int | model default | Number of denoising steps (higher = better quality, slower) |
+| `seed` | int | null | Random seed for reproducible generation |
+
+### Response Format
+
+Returns binary audio data with the appropriate `Content-Type` header:
+
+| `response_format` | Content-Type |
+|--------------------|--------------|
+| `wav` | `audio/wav` |
+| `mp3` | `audio/mpeg` |
+| `flac` | `audio/flac` |
+| `pcm` | `audio/pcm` |
+| `aac` | `audio/aac` |
+| `opus` | `audio/opus` |
+
+## Examples
+
+### Basic Generation
+
+Generate audio with only a text prompt (model defaults for all other parameters):
+
+```bash
+curl -X POST http://localhost:8000/v1/audio/generate \
+    -H "Content-Type: application/json" \
+    -d '{
+        "input": "The sound of ocean waves crashing on a beach"
+    }' --output ocean.wav
+```
+
+### Custom Duration
+
+Specify an explicit audio length in seconds:
+
+```bash
+curl -X POST http://localhost:8000/v1/audio/generate \
+    -H "Content-Type: application/json" \
+    -d '{
+        "input": "A dog barking",
+        "audio_length": 5.0
+    }' --output dog_5s.wav
+```
+
+### High Quality with Negative Prompt
+
+Use a negative prompt to steer generation away from undesired characteristics, and increase inference steps for higher quality:
+
+```bash
+curl -X POST http://localhost:8000/v1/audio/generate \
+    -H "Content-Type: application/json" \
+    -d '{
+        "input": "A piano playing a gentle melody",
+        "audio_length": 10.0,
+        "negative_prompt": "Low quality, distorted, noisy",
+        "guidance_scale": 8.0,
+        "num_inference_steps": 150
+    }' --output piano_hq.wav
+```
+
+### Reproducible Generation
+
+Set a `seed` to get deterministic results across runs:
+
+```bash
+curl -X POST http://localhost:8000/v1/audio/generate \
+    -H "Content-Type: application/json" \
+    -d '{
+        "input": "Thunder and rain sounds",
+        "audio_length": 15.0,
+        "seed": 42
+    }' --output thunder.wav
+```
+
+### Full Control
+
+Combine all parameters for precise control over generation:
+
+```bash
+curl -X POST http://localhost:8000/v1/audio/generate \
+    -H "Content-Type: application/json" \
+    -d '{
+        "input": "Thunder and rain sounds",
+        "audio_length": 15.0,
+        "negative_prompt": "Low quality",
+        "guidance_scale": 7.0,
+        "num_inference_steps": 100,
+        "seed": 42
+    }' --output thunder_rain.wav
+```
+
+### Quick Generation (Fewer Steps)
+
+For faster generation with slightly lower quality:
+
+```bash
+curl -X POST http://localhost:8000/v1/audio/generate \
+    -H "Content-Type: application/json" \
+    -d '{
+        "input": "Birds chirping in a forest",
+        "audio_length": 8.0,
+        "num_inference_steps": 50
+    }' --output birds_quick.wav
+```
+
+### Python Client
+
+```python
+import httpx
+
+response = httpx.post(
+    "http://localhost:8000/v1/audio/generate",
+    json={
+        "input": "Thunder and rain",
+        "audio_length": 15.0,
+        "negative_prompt": "Low quality",
+        "guidance_scale": 7.0,
+        "num_inference_steps": 100,
+        "seed": 42,
+        "response_format": "wav",
+    },
+    timeout=300.0,
+)
+
+with open("thunder.wav", "wb") as f:
+    f.write(response.content)
+```
+
+## Parameter Tuning Guide
+
+### `guidance_scale`
+
+Controls how closely the generated audio follows the text prompt.
+
+| Range | Behaviour |
+|-------|-----------|
+| 3 - 5 | More creative / varied output |
+| 7 (default) | Balanced adherence |
+| 10+ | Strict adherence to the prompt |
+
+### `num_inference_steps`
+
+Controls the number of denoising steps in the diffusion process.
+
+| Steps | Quality | Speed | Use Case |
+|-------|---------|-------|----------|
+| 50 | Good | Fast | Quick previews |
+| 100 | Very Good | Medium | General purpose |
+| 150+ | Excellent | Slow | Final / critical audio |
+
+### `audio_length`
+
+Duration of the generated audio clip. For `stable-audio-open-1.0`, the maximum is approximately 47 seconds. If omitted, the model uses its own default length.
+
+### `negative_prompt`
+
+Describes characteristics to avoid. Common negative prompts include:
+
+- `"Low quality, distorted, noisy"`
+- `"Silence, static"`
+- `"Music"` (when generating sound effects only)
+
+## Supported Models
+
+| Model | Description |
+|-------|-------------|
+| `stabilityai/stable-audio-open-1.0` | Open-source audio generation model, up to ~47 seconds, 44.1 kHz stereo |
+
+## Error Responses
+
+### 400 Bad Request
+
+Invalid or missing parameters:
+
+```json
+{
+    "error": {
+        "message": "Audio generation model did not produce audio output.",
+        "type": "BadRequestError",
+        "param": null,
+        "code": 400
+    }
+}
+```
+
+### 404 Not Found
+
+Model mismatch:
+
+```json
+{
+    "error": {
+        "message": "The model `xxx` does not exist.",
+        "type": "NotFoundError",
+        "param": "model",
+        "code": 404
+    }
+}
+```
+
+### 422 Unprocessable Entity
+
+Pydantic validation failure (e.g. invalid `response_format`, `speed` out of range):
+
+```json
+{
+    "detail": [
+        {
+            "type": "literal_error",
+            "msg": "Input should be 'wav', 'pcm', 'flac', 'mp3', 'aac' or 'opus'",
+            ...
+        }
+    ]
+}
+```
+
+## Troubleshooting
+
+### "Audio generation model did not produce audio output"
+
+The model finished but returned no audio data. Verify the server started successfully and the model loaded without errors.
+
+### Server Not Responding
+
+```bash
+# Check if the server is healthy
+curl http://localhost:8000/health
+```
+
+### Audio Quality Issues
+
+- Increase `num_inference_steps` (e.g. 150).
+- Add a negative prompt: `"Low quality, distorted, noisy"`.
+- Increase `guidance_scale` for stronger prompt adherence.
+
+### Generation Timeout
+
+- Reduce `num_inference_steps`.
+- Reduce `audio_length`.
+- Check GPU memory with `nvidia-smi`.
+
+### Out of Memory
+
+- Lower `--gpu-memory-utilization` (e.g. 0.8).
+- Reduce `audio_length`.
+
+## Development
+
+Enable debug logging:
+
+```bash
+vllm-omni serve stabilityai/stable-audio-open-1.0 \
+    --host 0.0.0.0 \
+    --port 8000 \
+    --gpu-memory-utilization 0.9 \
+    --trust-remote-code \
+    --enforce-eager \
+    --omni \
+    --uvicorn-log-level debug
+```