关于Colab上Keras模型转TPU模型的问题

使用TPU加速训练，将Keras模型转TPU模型时报错，如图

关键代码如下
引用库：

%tensorflow_version 1.x
import json
import os

import numpy as np
import tensorflow as tf
from  tensorflow.python.keras.applications import resnet
from  tensorflow.python.keras import callbacks
from  tensorflow.python.keras.preprocessing.image import ImageDataGenerator
import matplotlib.pyplot as plt

转换TPU模型代码如下

 # This address identifies the TPU we'll use when configuring TensorFlow.
        TPU_WORKER = 'grpc://' + os.environ['COLAB_TPU_ADDR']
        tf.logging.set_verbosity(tf.logging.INFO)

        self.model = tf.contrib.tpu.keras_to_tpu_model(self.model, strategy=tf.contrib.tpu.TPUDistributionStrategy(tf.contrib.cluster_resolver.TPUClusterResolver(TPU_WORKER)))
        self.model = resnet50.ResNet50(weights=None, input_shape=dataset.input_shape, classes=num_classes)

https://blog.csdn.net/big91987/article/details/87898100