title: error in using concatenate in keras in google colab
body: <p><img src="https://i.stack.imgur.com/4TC8z.png" alt="the_error_i_received"></p>
  
  <p>the error is mentioned in the picture.</p>
  
  <pre><code>TypeError                                 Traceback (most recent call last)
  &lt;ipython-input-200-8ea67f851d87&gt; in &lt;module&gt;()
        1 final_model = Sequential([
  ----&gt; 2   Concatenate([image_model, caption_model], mode='concat', concat_axis=1),
        3   Bidirectional(LSTM(256, return_sequences=False)),
        4   Dense(vocab_size),
        5   Activation('softmax')
  </code></pre>