title: module 'tensorflow.python.keras.api._v2.keras.layers' has no attribute 'CuDNNLSTM'
body: <p>When I write <code>tf.keras.layers.LSTM</code>, I get the warning </p>
  
  <p><code>Note that this layer is not optimized for performance. Please use tf.keras.layers.CuDNNLSTM for better performance on GPU.</code></p>
  
  <p>But when I change the layer to <code>tf.keras.layers.CuDNNLSTM</code>, I get the error </p>
  
  <p><code>AttributeError: module 'tensorflow.python.keras.api._v2.keras.layers' has no attribute 'CuDNNLSTM'</code></p>
  
  <p>Tensorflow version is 2.0.0-alpha0, Keras version is 2.2.4-tf.</p>
  
  <p>How can I fix this problem?</p>