为什么我会得到"You must feed a value for placeholder tensor 'output' with dtype int64"？

我正在尝试构建一个C++程序，该程序读取预先训练的模型并使用它。我从这里获取了代码并对其进行了一点修改。我现在拥有的是：

int main(int argc, char* argv[]) {
  // Initialize a tensorflow session
  Session* session;
  Status status = NewSession(SessionOptions(), &session);
  if (!status.ok()) {
    std::cout << status.ToString() << "n";
    return 1;
  }
  // Read in the protobuf graph we exported
  GraphDef graph_def;
  status = ReadTextProto(Env::Default(), "models/train.pbtxt", &graph_def);
  if (!status.ok()) {
    std::cout << status.ToString() << "n";
    return 1;
  }
  // Add the graph to the session
  status = session->Create(graph_def);
  if (!status.ok()) {
    std::cout << status.ToString() << "n";
    return 1;
  }
  tensorflow::Tensor inputs(DT_FLOAT, TensorShape({46}));
  auto inputs_flat = inputs.flat<float>();
  inputs_flat.setRandom();
  // The session will initialize the outputs
  std::vector<tensorflow::Tensor> outputs;
  status = session->Run({{"input", inputs}}, {"output"}, {}, &outputs);
  if (!status.ok()) {
    std::cout << status.ToString() << "n";  // <--- error shows here
    return 1;
  }
  // Grab the first output
  // and convert the node to a scalar representation.
  auto output_c = outputs[0].scalar<int>();
  // Print the results
  std::cout << outputs[0].DebugString() << "n";
  std::cout << output_c() << "n";
  // Free any resources used by the session
  session->Close();
  return 0;
}

但是当我运行它时，我得到

Invalid argument: You must feed a value for placeholder tensor 'output' with dtype int64
     [[Node: output = Placeholder[_output_shapes=[[-1]], dtype=DT_INT64, shape=[], _device="/job:localhost/replica:0/task:0/cpu:0"]()]]

我在models/train.pbtxt中阅读的图表有 14K 行，所以我不在这里复制它。我将把相关部分：

...................
node {
  name: "input"
  op: "Placeholder"
  attr {
    key: "_output_shapes"
    value {
      list {
        shape {
          dim {
            size: -1
          }
          dim {
            size: 46
          }
        }
      }
    }
  }
  attr {
    key: "dtype"
    value {
      type: DT_FLOAT
    }
  }
  attr {
    key: "shape"
    value {
      shape {
      }
    }
  }
}
node {
  name: "output"
  op: "Placeholder"
  attr {
    key: "_output_shapes"
    value {
      list {
        shape {
          dim {
            size: -1
          }
        }
      }
    }
  }
  attr {
    key: "dtype"
    value {
      type: DT_INT64
    }
  }
  attr {
    key: "shape"
    value {
      shape {
      }
    }
  }
}
...................

所以阅读问题：此错误消息告诉我什么？