title: what is the meaning of negative dimension in keras theano
body: <p>I've tried define a simple architecture to use on MNIST dataset , I start defined my architecture like this :</p>
  
  <pre><code>model = Sequential()
  
  model.add(Convolution2D(32,(3,3),activation='relu',input_shape = (1,28,28)));
  </code></pre>
  
  <p>in this step I checked my network out put :</p>
  
  <pre><code>model.output_shape
  </code></pre>
  
  <p>it gives me this :</p>
  
  <pre><code>(None, -1, 26, 32)
  </code></pre>
  
  <p>could someone explain to me what is the meaning of negative dimension (-1) ?</p>