title: AttributeError: 'Model' object has no attribute '_name' during input layer concatenation
body: <p>I cannot understand what is wrong. I need to have two sets of inputs so i divided them to give each one a name (serving purposes) and then concatenated them to link them to the next layer.</p>
  
  <pre><code>layer_input1  = tf.keras.Input(shape=(None, 1), name='layer1')
  layer_input2  = tf.keras.Input(shape=(None, 1), name='layer2')
  
  layer_input = tf.keras.layers.concatenate([layer_input1, layer_input2], name='inputs')
  fc_1 = tf.keras.layers.Dense(2,
                               activation='relu')(layer_input)
  fc_1 = tf.keras.layers.Dropout(0.5)(fc_1)
  fc_2 = tf.keras.layers.Dense(10,
                               activation='relu')(fc_1)
  output_layer = tf.keras.layers.Dense(1,
                               activation='relu', name='predictions')(fc_2)
  model = tf.keras.Model(inputs=layer_input, outputs=output_layer)
  
  AttributeError                            Traceback (most recent call last)
  &lt;ipython-input-430-b567199137e0&gt; in &lt;module&gt;()
       10 output_layer = tf.keras.layers.Dense(1,
       11                              activation='relu', name='predictions')(fc_2)
  ---&gt; 12 model = tf.keras.Model(inputs=layer_input, outputs=output_layer)
  
  AttributeError: 'Model' object has no attribute '_name'
  </code></pre>