[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/design/feature/diffusion_step_execution.md

@@ -1,4 +1,4 @@
-# Diffusion Step Execution
+# Adding Step Execution Support for Diffusion Pipelines
 
 This guide documents vLLM-Omni's stepwise diffusion contract for model authors
 and contributors implementing `step_execution=True` support for a diffusion
@@ -7,12 +7,20 @@ pipeline.
 For end-user enablement, supported models, and current limitations, see
 [Step Execution](../../user_guide/diffusion/step_execution.md).
 
+This document describes the base step-execution contract only. For the
+experimental batching policy layered on top of the step-wise path, see
+[Continuous Batching for Step-Wise Diffusion](diffusion_continuous_batching.md).
+
 ## Current Support Scope
 
 `step_execution` is **not** a generic diffusion toggle. It only works for
 pipelines that implement the segmented stateful contract in
 [`vllm_omni/diffusion/models/interface.py`](gh-file:vllm_omni/diffusion/models/interface.py).
 
+This page is intentionally author-facing. Treat runtime enablement
+(`step_execution=True` in Python or `--step-execution` in serving) as an
+opt-in user knob layered on top of the implementation contract below.
+
 Current in-tree support:
 
 | Pipeline | Example models | Step execution |
@@ -22,7 +30,9 @@ Current in-tree support:
 
 Current engine/runtime limitations:
 
-- `StepScheduler` only schedules `batch_size=1`.
+- Continuous batching with `max_num_seqs > 1` is experimental and documented in
+  [Continuous Batching for Step-Wise Diffusion](diffusion_continuous_batching.md).
+  Keep `max_num_seqs=1` if you want the older conservative behavior.
 - `cache_backend` is not supported in step mode.
 - Request-mode extras such as KV transfer are not wired into step mode yet.
 - Unsupported pipelines now fail early during model loading instead of failing on the first request.