title: tensorflow select list of indices along dimension
body: <p>In order to select a list of columns in a matrix I am doing the following:</p>
  
  <pre><code>sel = tf.concat([tf.slice(mat, [0, i], [-1, 1]) for i in list_columns],
                  axis=1)
  </code></pre>
  
  <p>I wonder if there is a more efficient manner</p>