Add type check

python/paddle/v2/fluid/clip.py

@@ -40,6 +40,11 @@ def error_clip_callback(block, context):
                          op_desc.output_arg_names()):
         fwd_var = block.var_recursive(grad_to_var[grad_n])
         error_clip = getattr(fwd_var, "error_clip", None)
+        if not (error_clip is None or isinstance(error_clip,
+                                                 BaseErrorClipAttr)):
+            raise TypeError(
+                "Variable's error_clip should be an instance of BaseErrorClipAttr or None."
+            )
         if error_clip is not None:
             error_clip.append_clip_op(block, grad_n)