title: Tensorflow Dictionary lookup with String tensor
body: <p>Is there any way to perform a dictionary lookup based on a String tensor in Tensorflow?</p>
  
  <p>In plain Python, I'd do something like</p>
  
  <pre><code>value = dictionary[key]
  </code></pre>
  
  <p>. Now I'd like to do the same thing at Tensorflow runtime, when I have my <code>key</code> as a String tensor. Something like</p>
  
  <pre><code>value_tensor = tf.dict_lookup(string_tensor)
  </code></pre>
  
  <p>would be nice.</p>