title: In tensorflow, randomly (sub)sample k entries from a tensor along 0-axis
body: <p>Given a tensor of <code>rank&gt;=1</code> <code>T</code>, I would like to randomly sample <code>k</code> entries from it, uniformly, along the 0-axis.</p>
  
  <p>EDIT: <strong>The sampling should be part of the computation graph as a lazy operation and should output different random entries every time it is called.</strong></p>
  
  <p>For instance, given <code>T</code> of rank <code>2</code>:</p>
  
  <pre><code>T = tf.constant( \
       [[1,1,1,1,1],
        [2,2,2,2,2],
        [3,3,3,3,3],
        ....
        [99,99,99,99,99],
        [100,100,100,100,100]] \
       )
  </code></pre>
  
  <p>With <code>k=3</code>, a possible output would be:</p>
  
  <pre><code>#output = \
  #   [[34,34,34,34,34],
  #    [6,6,6,6,6],
  #    [72,72,72,72,72]]
  </code></pre>
  
  <p>How can this be achieved in tensorflow?</p>