title: Element-wise assignment in tensorflow
body: <p>In <code>numpy</code>, it could be easily done as</p>
  
  <pre><code>&gt;&gt;&gt; img
  array([[1, 2, 3],
         [4, 5, 6],
         [7, 8, 9]], dtype=int32)
  &gt;&gt;&gt; img[img&gt;5] = [1,2,3,4]
  &gt;&gt;&gt; img
  array([[1, 2, 3],
         [4, 5, 1],
         [2, 3, 4]], dtype=int32)
  </code></pre>
  
  <p>However, there seems not exist similar operation in tensorflow.</p>