[Bugfix] Fix generation artifacts of Qwen-Image-Edit-2511 and update pipeline DiT param parsing (#776)

Signed-off-by: samithuang <285365963@qq.com>

vllm_omni/diffusion/diffusion_engine.py

@@ -293,6 +293,7 @@ class DiffusionEngine:
     def _dummy_run(self):
         """A dummy run to warm up the model."""
         prompt = "dummy run"
+        # note that num_inference_steps=1 will cause timestep and temb None in the pipeline
         num_inference_steps = 1
         height = 1024
         width = 1024