code: def convert_model_to_onnx(frozen_graph_path, end_node_names, onnx_output_path):
      
      with tf.gfile.GFile(frozen_graph_path, "rb") as f:
          graph_def = tf.GraphDef()
          graph_def.ParseFromString(f.read())
          onnx_model = tensorflow_graph_to_onnx_model(graph_def, end_node_names, opset=6)
          file = open(onnx_output_path, "wb")
          file.write(onnx_model.SerializeToString())
          file.close()
docs: Reimplementation of the TensorFlow-onnx official tutorial convert the proto buff to onnx file:
  
      Parameters
      -----------
      frozen_graph_path : string
          the path where your frozen graph file save.
      end_node_names : string
          the name of the end node in your graph you want to get in your proto buff
      onnx_output_path : string
          the path where you want to save the onnx file.
  
      References
      -----------
      - `onnx-tf exporting tutorial <https://github.com/onnx/tutorials/blob/master/tutorials/OnnxTensorflowExport.ipynb>`