Fix bug

Author: reyoung

python/paddle/fluid/backward.py

@@ -449,6 +449,17 @@ def append_backward(loss, parameter_list=None, no_grad_set=None,
     """
     assert isinstance(loss, framework.Variable)
 
+    if loss.op is None:
+        # the loss is from a cloned program. Find loss op manually.
+        for op in reversed(loss.block.ops):
+            assert isinstance(op, framework.Operator)
+            if len(op.output_arg_names) == 1 and op.output_arg_names[
+                    0] == loss.name:
+                loss.op = op
+                break
+        if loss.op is None:
+            raise ValueError("loss.op is None. Should not happend")
+
     loss.op.set_attr(core.op_proto_and_checker_maker.kOpRoleAttrName(),
                      int(core.op_proto_and_checker_maker.OpRole.Forward) |
                      int(core.op_proto_and_checker_maker.OpRole.Loss))