add check if the model does not save model

test=develop

Author: jacquesqiao

python/paddle/fluid/tests/book/test_label_semantic_roles.py

@@ -208,6 +208,10 @@ def train_loop(main_program):
 
                 batch_id = batch_id + 1
 
+        raise RuntimeError(
+            "This model should save_inference_model and return, but not reach here, please check!"
+        )
+
     if is_local:
         train_loop(fluid.default_main_program())
     else: