fix detector evaluate bug

dygraph/paddlex/cv/models/detector.py

@@ -419,6 +419,9 @@ class BaseDetector(BaseModel):
             if return_details:
                 return scores, self.eval_details
             return scores
+        if return_details:
+            return None, None
+        return None
 
     def predict(self, img_file, transforms=None):
         """