[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>

benchmarks/diffusion/performance_dashboard/qwen_image_serving_performance.md

@@ -33,6 +33,12 @@ vllm serve Qwen/Qwen-Image --omni \
     --port 8091
 ```
 
+To replay step-wise continuous batching, compare
+`vllm serve Qwen/Qwen-Image --omni --port 8089 --step-execution --max-num-seqs 1`
+against the same command with `--max-num-seqs 8`. `--step-execution` is the
+feature gate, and increasing `--max-num-seqs` above `1` lets the scheduler
+keep more compatible requests active at once.
+
 ## 3.2 Key Parameters
 
 | Parameter             | Description              |
@@ -70,6 +76,7 @@ python benchmarks/diffusion/diffusion_benchmark_serving.py \
 | `--task`               | Task type (e.g., `t2i`)           |
 | `--num-prompts`        | Total number of requests          |
 | `--max-concurrency`    | Client-side concurrency           |
+| `--warmup-concurrency` | Warmup concurrency for batch-shape prewarming |
 | `--random-request-config`| JSON string defining random request |
 
 ---
@@ -133,6 +140,12 @@ python benchmarks/diffusion/diffusion_benchmark_serving.py \
     ]'
 ```
 
+For a continuous-batching A/B replay, run the same benchmark command twice
+against the two serving configs above and keep traffic plus warmup settings
+identical across both runs. If the measured run uses `--max-concurrency 8`,
+warm with `--warmup-requests 8 --warmup-concurrency 8` so the first measured
+batch does not include compile or CUDA-graph capture overhead.
+
 ---
 
 # 6. Performance Metrics
@@ -163,6 +176,7 @@ To ensure consistent and comparable benchmark results:
 * Record parallel configuration
 * Record benchmark parameters (resolution, concurrency, number of prompts)
 * Ensure no background workload on GPUs during testing
+* When comparing continuous batching, keep warmup settings the same across runs
 
 ---