title: swap tensor axis in keras
body: <p>I want to swap tensor axis of image batches from (batch_size, row, col, ch) to 
  (batch_size, ch, row, col). </p>
  
  <p>in numpy, this can be done with </p>
  
  <pre><code>X_batch = np.moveaxis( X_batch, 3, 1)
  </code></pre>
  
  <p>How would I do that in Keras? </p>