[Feat] DiffusionEngine Support async batch infer (#2729)

Signed-off-by: Semmer <semmer@live.cn>
Signed-off-by: jader <yjader@foxmail.com>
Signed-off-by: asukaqaq-s <1311722138@qq.com>
Co-authored-by: asukaqaq-s <1311722138@qq.com>
Co-authored-by: jader <yjader@foxmail.com>
Co-authored-by: Hongsheng Liu <liuhongsheng4@huawei.com>
Co-authored-by: Gao Han <hgaoaf@connect.ust.hk>

docs/user_guide/diffusion/step_execution.md

@@ -8,6 +8,8 @@ implement the stepwise contract support it today.
 
 ## Quick Start
 
+### Python API
+
 ```python
 from vllm_omni import Omni
 from vllm_omni.inputs.data import OmniDiffusionSamplingParams
@@ -25,6 +27,18 @@ outputs = omni.generate(
 )
 ```
 
+### Serving
+
+```bash
+vllm serve Qwen/Qwen-Image --omni \
+  --port 8091 \
+  --step-execution \
+  --max-num-seqs 8
+```
+
+For serving, `--step-execution` enables the step-wise runtime. Continuous
+batching only becomes relevant when `--max-num-seqs > 1`.
+
 ## Supported Pipelines
 
 | Pipeline | Example models | Step execution |
@@ -32,9 +46,17 @@ outputs = omni.generate(
 | `QwenImagePipeline` | `Qwen/Qwen-Image`, `Qwen/Qwen-Image-2512` | Yes |
 | All other diffusion pipelines | `QwenImageEditPipeline`, `QwenImageEditPlusPipeline`, `QwenImageLayeredPipeline`, GLM-Image, Wan, Flux, etc. | No |
 
+!!! warning "Experimental continuous batching"
+    When `--step-execution` is enabled and `max_num_seqs > 1` is configured,
+    the step-wise path can batch
+    compatible requests together. This is experimental. Requests with
+    incompatible sampling parameters are intentionally kept in separate batches,
+    and `max_num_seqs=1` remains the conservative default.
+
 ## Current Limitations
 
-- `step_execution` currently supports `batch_size=1` only.
+- Continuous batching under `step_execution` is experimental and only batches
+  compatible requests.
 - `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.
@@ -45,6 +67,11 @@ 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.
 
+For Qwen-Image online serving, the usual progression is:
+
+- start with `--step-execution --max-num-seqs 1` if you only need the step-wise path
+- increase `--max-num-seqs` after that if you want the experimental compatible-request batching behavior
+
 If you are looking for general diffusion speedups, see
 [Diffusion Features Overview](../diffusion_features.md).
 
@@ -59,3 +86,7 @@ pipeline does not support step execution.
 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).
+
+If you also want that pipeline to participate in the experimental batched
+step-wise path, see:
+[Continuous Batching for Step-Wise Diffusion](../../design/feature/diffusion_continuous_batching.md).