title: tensorflow merge and zip two tensors
body: <p>I have two tensors as below:</p>
  
  <pre><code>x1 = tf.constant([[1.0, 2.0, 3.0], [4.0, 5.0, 6.0]])
  x2 = tf.constant([[7.0, 8.0, 9.0], [10.0, 11.0, 12.0]])
  </code></pre>
  
  <p>How should I merge and transform x1 and x2, so that I can have a tensor like below:</p>
  
  <pre><code>[[[1.0, 7.0]
    [2.0, 8.0]
    [3.0, 9.0]]
  
   [[4.0, 10.0]
    [5.0, 11.0]
    [6.0, 12.0]]
  ]
  </code></pre>