reafator pipeline stage/step pipeline (#1368)

Signed-off-by: asukaqaq-s <1311722138@qq.com>

docs/user_guide/diffusion/step_execution.md

@@ -0,0 +1,61 @@
+# Step Execution
+
+Step execution is an opt-in diffusion execution mode enabled with
+`step_execution=True` when constructing `Omni`.
+
+It is not a generic diffusion toggle for every pipeline. Only pipelines that
+implement the stepwise contract support it today.
+
+## Quick Start
+
+```python
+from vllm_omni import Omni
+from vllm_omni.inputs.data import OmniDiffusionSamplingParams
+
+omni = Omni(
+    model="Qwen/Qwen-Image",
+    step_execution=True,
+)
+
+outputs = omni.generate(
+    "A cat sitting on a windowsill",
+    OmniDiffusionSamplingParams(
+        num_inference_steps=50,
+    ),
+)
+```
+
+## Supported Pipelines
+
+| Pipeline | Example models | Step execution |
+|----------|----------------|----------------|
+| `QwenImagePipeline` | `Qwen/Qwen-Image`, `Qwen/Qwen-Image-2512` | Yes |
+| All other diffusion pipelines | `QwenImageEditPipeline`, `QwenImageEditPlusPipeline`, `QwenImageLayeredPipeline`, GLM-Image, Wan, Flux, etc. | No |
+
+## Current Limitations
+
+- `step_execution` currently supports `batch_size=1` only.
+- `cache_backend` is not supported together with step execution.
+- Unsupported pipelines fail early during model loading.
+- Request-mode extras such as KV transfer are not wired into step mode yet.
+
+## When To Use It
+
+Use step execution only when you specifically need the pipeline to run through
+its stepwise request state machine. For normal diffusion inference, leave it
+disabled unless your workflow depends on this mode.
+
+If you are looking for general diffusion speedups, see
+[Diffusion Acceleration Overview](../diffusion_acceleration.md).
+
+## Troubleshooting
+
+If model loading fails with a message mentioning `prepare_encode()`,
+`denoise_step()`, `step_scheduler()`, and `post_decode()`, the selected
+pipeline does not support step execution.
+
+## For Model Authors
+
+If you want to add step execution support to a new diffusion pipeline, see the
+implementation guide:
+[Diffusion Step Execution Design](../../design/feature/diffusion_step_execution.md).