title: tensorflow lstm convert to tensor tpu model?
body: <p>what issue can be?</p>
  
  <pre><code>from keras.models import Sequential
  from keras.layers import Dense, Dropout, Activation, Input, LSTM, Dense
  def create_model():
  # create a small LSTM network
      model = Sequential()
      model.add(LSTM(20, input_shape=(X.shape[1], X.shape[2]), return_sequences=True))
      model.add(LSTM(20, return_sequences=True))
      model.add(LSTM(10, return_sequences=True))
      model.add(Dropout(0.2))
      model.add(LSTM(4, return_sequences=False))
      model.add(Dense(4, kernel_initializer='uniform', activation='relu'))
      model.add(Dense(1, kernel_initializer='uniform', activation='relu'))
  
  import tensorflow as tf
  
  tf.logging.set_verbosity(tf.logging.DEBUG)
  
  tf.keras.backend.clear_session()
  model=create_model()
  
  tpu_model = tf.contrib.tpu.keras_to_tpu_model(
      model,
      strategy=tf.contrib.tpu.TPUDistributionStrategy(
          tf.contrib.cluster_resolver.TPUClusterResolver(TPU_WORKER)))
  </code></pre>
  
  <blockquote>
    <p>WARNING:tensorflow:tpu_model (from tensorflow.contrib.tpu.python.tpu.keras_support) is experimental and may change or be removed at any time, and without warning.</p>
    
    <p>ValueError: Fetch argument  cannot be interpreted as a Tensor. (Tensor Tensor("lstm_13/kernel:0", shape=(79, 320), dtype=float32_ref) is not an element of this graph.)</p>
  </blockquote>