[diffusion][bugfix] Separate Diffusers hooks from model metadata (#6749)

Signed-off-by: david6666666 <530634352@qq.com>
Signed-off-by: Gao Han <hgaoaf@connect.ust.hk>
Co-authored-by: Gao Han <hgaoaf@connect.ust.hk>

vllm_omni/diffusion/data.py

@@ -685,6 +685,19 @@ def resolve_model_class_name(
     return None
 
 
+def uses_diffusers_adapter(od_config: object) -> bool:
+    """Return whether execution uses the Diffusers adapter backend.
+
+    A custom pipeline takes precedence over ``diffusion_load_format`` in the
+    worker, so adapter-specific behavior must only apply when no custom
+    pipeline override is configured.
+    """
+    return (
+        getattr(od_config, "custom_pipeline_args", None) is None
+        and getattr(od_config, "diffusion_load_format", "default") == "diffusers"
+    )
+
+
 @dataclass
 class OmniDiffusionConfig:
     # Model and path configuration (for convenience)
@@ -1317,10 +1330,6 @@ class OmniDiffusionConfig:
 
         from vllm_omni.diffusion.utils.hf_utils import get_diffusion_model_index
 
-        # Default model_class_name for diffusers adapter
-        if self.model_class_name is None and self.diffusion_load_format == "diffusers":
-            self.model_class_name = "DiffusersAdapterPipeline"
-
         assert self.model is not None
         try:
             config_dict = get_diffusion_model_index(
@@ -1330,6 +1339,8 @@ class OmniDiffusionConfig:
             if config_dict is not None:
                 if self.model_class_name is None:
                     self.model_class_name = config_dict.get("_class_name", None)
+                    if self.model_class_name is None and self.diffusion_load_format == "diffusers":
+                        self.model_class_name = "DiffusersAdapterPipeline"
                 self.update_multimodal_support()
 
                 # Skip transformer config loading for diffusers adapter
@@ -1370,6 +1381,8 @@ class OmniDiffusionConfig:
             # Skip transformer config loading for diffusers adapter
             # (non-DiT models don't have a separate transformer folder/config)
             if self.diffusion_load_format == "diffusers":
+                if self.model_class_name is None:
+                    self.model_class_name = "DiffusersAdapterPipeline"
                 self.set_tf_model_config(TransformerConfig())
                 logger.warning(
                     "Could not find a valid pipeline index per Diffusers format. "