title: Tensorflow: How to tile a tensor that duplicate in certain order?
body: <p>For example, I have a tensor <code>A = tf.Variable([a, b, c, d, e])</code> and through 
  <code>tf.tile()</code>, it can give a tensor like <code>[a, b, c, d, e, a, b, c, d, e]</code></p>
  
  <p>But I want to reform <code>A</code> into something like: <code>[a, a, b, b, c, c, d, d, e]</code>, where the elements are duplicated at the original place.</p>
  
  <p>What is the <strong>most efficient way (less operations)</strong> to achieve that (through different-able ops)?</p>