[Refactor] Remove the default value "mp" from distributed_executor_backend. (#3007)

Signed-off-by: amy-why-3459 <wuhaiyan17@huawei.com>

vllm_omni/diffusion/executor/abstract.py

@@ -22,6 +22,10 @@ class DiffusionExecutor(ABC):
     def get_class(od_config: OmniDiffusionConfig) -> type[DiffusionExecutor]:
         executor_class: type[DiffusionExecutor]
         distributed_executor_backend = od_config.distributed_executor_backend
+        # Keep backward-compatible behavior for callers/configs that omit this
+        # field and rely on the historical diffusion default backend.
+        if distributed_executor_backend is None:
+            distributed_executor_backend = "mp"
 
         if isinstance(distributed_executor_backend, type):
             if not issubclass(distributed_executor_backend, DiffusionExecutor):