title: Keras model not works well on TPU since June 14
body: <p>I notice that this code cannot work since June 14 as it says: 'Model' object has no attribute 'target_tensors'. </p>
  
  <p>I change another way to use the TPU distribution strategy following <a href="https://colab.research.google.com/github/tensorflow/docs/blob/master/site/en/guide/distribute_strategy.ipynb#scrollTo=eQ1QESxxEbCh" rel="noreferrer">https://colab.research.google.com/github/tensorflow/docs/blob/master/site/en/guide/distribute_strategy.ipynb#scrollTo=eQ1QESxxEbCh</a></p>
  
  <p>But in this way, I need to set a validation_fre parameter, which significantly slows down the overall training process.  How could I deal with that problem?</p>
  
  <pre class="lang-py prettyprint-override"><code>resolver = tf.contrib.cluster_resolver.TPUClusterResolver('grpc://' + os.environ['COLAB_TPU_ADDR'])
  tf.contrib.distribute.initialize_tpu_system(resolver)
  strategy = tf.contrib.distribute.TPUStrategy(resolver)
  with strategy.scope():
   model=myKerasmodel()
   model.compile()....
  
  
  </code></pre>