title: Tensorflow: How can I use tf.roll without wrapping?
body: <p>I want to independently shift the columns or rows of a 2-D tensor like:</p>
  <pre><code>a = tf.constant([[1,2,3], [4,5,6]])
  shift = tf.constant([2, -1])
  b = shift_fn(a, shift)
  </code></pre>
  <p>which gives me:</p>
  <pre><code>b = [[0, 0, 1], [5, 6, 0]]
  </code></pre>
  <p>I find that <code>tf.roll()</code> can do similar things but will wrap the elements. How can I pad zeros using it?</p>