问tensorflow.lite.python.convert.ConverterError？

import tensorflow as tf
import tensorflow.compat.v1 as tf
tf.disable_v2_behavior()



# 把pb文件路径改成自己的pb文件路径即可
path = "../model/HTOCR.pb"

# 如果是不知道自己的模型的输入输出节点，建议用tensorboard做可视化查看计算图，计算图里有输入输出的节点名称
inputs = ["Placeholder","Placeholder_4","is_train"]
outputs = ["CTCGreedyDecoder","transpose"]

input_tensor_shape = {'Placeholder': [1, 616, 32],'Placeholder_4':[1],'is_train':[1]}
# 转换pb模型到tflite模型
converter = tf.lite.TFLiteConverter.from_frozen_graph(path, inputs, outputs,input_tensor_shape)
# converter.post_training_quantize = True

tflite_model = converter.convert()
open("../model/HTOCR.tflite", "wb").write(tflite_model)