title: How to stack multiple lstm in keras?
body: <p>I am using deep learning library keras and trying to stack multiple LSTM with no luck.
  Below is my code</p>
  
  <pre><code>model = Sequential()
  model.add(LSTM(100,input_shape =(time_steps,vector_size)))
  model.add(LSTM(100))
  </code></pre>
  
  <p>The above code returns error in the third line <code>Exception: Input 0 is incompatible with layer lstm_28: expected ndim=3, found ndim=2
  </code></p>
  
  <p>The input X is a tensor of shape (100,250,50). I am running keras on tensorflow backend</p>