Add Fish Speech S2 Pro support with online serving and voice cloning (#1798)

Signed-off-by: linyueqian <linyueqian@outlook.com>

docs/user_guide/examples/offline_inference/fish_speech.md

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+# Fish Speech S2 Pro
+
+Source <https://github.com/vllm-project/vllm-omni/tree/main/examples/offline_inference/fish_speech>.
+
+
+This directory contains an offline demo for running Fish Speech S2 Pro (`fishaudio/s2-pro`) with vLLM Omni. It supports text-only synthesis and voice cloning with reference audio.
+
+## Model Overview
+
+[Fish Speech S2 Pro](https://huggingface.co/fishaudio/s2-pro) is a 4B dual-AR text-to-speech model by FishAudio. It uses:
+
+- **Slow AR**: Generates semantic tokens autoregressively (Qwen3-based backbone)
+- **Fast AR**: Predicts residual codebook tokens from semantic tokens
+- **DAC Decoder**: Converts codec codes to 44.1 kHz audio waveform
+
+The model produces high-quality speech with voice cloning capabilities.
+
+## Prerequisites
+
+Install the `fish-speech` package for the DAC codec:
+
+```bash
+pip install fish-speech
+```
+
+## Setup
+
+Please refer to the [stage configuration documentation](https://docs.vllm.ai/projects/vllm-omni/en/latest/configuration/stage_configs/) to configure memory allocation appropriately for your hardware setup.
+
+## Quick Start
+
+### Text-Only Synthesis
+
+```bash
+python end2end.py --text "Hello, this is a test of the Fish Speech text to speech system."
+```
+
+Generated audio files are saved to `output_audio/` by default.
+
+### Voice Cloning
+
+Provide a reference audio file and its transcript to clone a voice:
+
+```bash
+python end2end.py \
+    --text "Hello, this is a cloned voice." \
+    --ref-audio /path/to/reference.wav \
+    --ref-text "Transcript of the reference audio."
+```
+
+## Streaming Mode
+
+Add `--streaming` to stream audio chunks progressively via `AsyncOmni` (requires `async_chunk: true` in the stage config):
+
+```bash
+python end2end.py --text "Hello, this is a test." --streaming
+```
+
+Each DAC decoder chunk is logged as it arrives. The final WAV file is written once generation completes.
+
+## Configuration
+
+| Argument | Default | Description |
+|----------|---------|-------------|
+| `--text` | `"Hello, this is a test..."` | Text to synthesize |
+| `--ref-audio` | None | Path to reference audio for voice cloning |
+| `--ref-text` | None | Transcript of the reference audio |
+| `--model` | `fishaudio/s2-pro` | HuggingFace model path |
+| `--stage-configs-path` | `fish_speech_s2_pro.yaml` | Path to stage configs YAML |
+| `--output-dir` | `output_audio` | Output directory for WAV files |
+| `--streaming` | False | Enable streaming via AsyncOmni |
+| `--stage-init-timeout` | 600 | Stage initialization timeout (seconds) |
+
+## Notes
+
+- Output audio is 44.1 kHz mono WAV.
+- The DAC codec weights (`codec.pth`) are loaded lazily from the model directory.
+- Voice cloning encodes the reference audio using the DAC codec to extract semantic codes, then prepends them as a system message.
+
+## Example materials
+
+??? abstract "diag_codebook.py"
+    ``````py
+    --8<-- "examples/offline_inference/fish_speech/diag_codebook.py"
+    ``````
+??? abstract "diag_reference_compare.py"
+    ``````py
+    --8<-- "examples/offline_inference/fish_speech/diag_reference_compare.py"
+    ``````
+??? abstract "end2end.py"
+    ``````py
+    --8<-- "examples/offline_inference/fish_speech/end2end.py"
+    ``````
+??? abstract "standalone_test.py"
+    ``````py
+    --8<-- "examples/offline_inference/fish_speech/standalone_test.py"
+    ``````