fix Serial output type

paddle/fluid/operators/checkpoint_save_op.cc

@@ -87,7 +87,7 @@ class CheckpointSaveOp : public framework::OperatorBase {
     std::string *serial_num = serial_var->GetMutable<std::string>();
     serial_num->append("0");
     dir.append("/");
-    dir.append(serial_num);
+    dir.append(serial_num->c_str());
     MkDirRecursively(dir.c_str());
 
     auto inp_var_names = Inputs("X");
@@ -159,10 +159,29 @@ to a file on disk.
   }
 };
 
+class CheckpointSaveOpVarTypeInference : public framework::VarTypeInference {
+ public:
+  void operator()(const framework::OpDesc &op_desc,
+                  framework::BlockDesc *block) const override {
+    auto out_var_name = op_desc.Output("Serial").front();
+    auto &out_var = block->FindRecursiveOrCreateVar(out_var_name);
+    auto var_type = framework::proto::VarType::RAW;
+    out_var.SetType(var_type);
+  }
+};
+
+class CheckpointSaveOpShapeInference : public framework::InferShapeBase {
+ public:
+  void operator()(framework::InferShapeContext *ctx) const override {}
+};
+
 }  // namespace operators
 }  // namespace paddle
 
 namespace ops = paddle::operators;
 
-REGISTER_OPERATOR(checkpoint_save, ops::CheckpointSaveOp,
-                  ops::CheckpointSaveOpProtoMaker);
+REGISTER_OPERATOR(send_vars, ops::CheckpointSaveOp,
+                  paddle::framework::EmptyGradOpMaker,
+                  ops::CheckpointSaveOpProtoMaker,
+                  ops::CheckpointSaveOpVarTypeInference,
+                  ops::CheckpointSaveOpShapeInference);