[Bugfix] fix diffusion quantization benchmarking for Omni outputs (#3653)

Signed-off-by: Lancer <maruixiang6688@gmail.com>

benchmarks/diffusion/quantization_quality.py

@@ -148,6 +148,7 @@ def _build_omni_kwargs(args, quantization=None):
 def _generate_image(omni, args, prompt, seed):
     """Generate a single image and return (PIL.Image, time_seconds, memory_gib)."""
     from vllm_omni.inputs.data import OmniDiffusionSamplingParams
+    from vllm_omni.outputs import OmniRequestOutput
     from vllm_omni.platforms import current_omni_platform
 
     generator = torch.Generator(device=current_omni_platform.device_type).manual_seed(seed)
@@ -165,8 +166,9 @@ def _generate_image(omni, args, prompt, seed):
     elapsed = time.perf_counter() - start
     peak_mem = torch.accelerator.max_memory_allocated() / (1024**3)
 
-    first = outputs[0]
-    req_out = first.request_output[0] if hasattr(first, "request_output") else first
+    req_out = OmniRequestOutput.unwrap_result(outputs)
+    if not req_out.images:
+        raise ValueError("Could not extract image output from result.")
     img = req_out.images[0]
     return img, elapsed, peak_mem