title: Setting num_epochs on tf.train.string_input_producer produces an error
body: <p>Setting num_epochs on tf.train.string_input_producerto anything other than None produces the error</p>
  
  <pre><code>Attempting to use uninitialized value input_producer/limit_epochs/epoch
  </code></pre>
  
  <p>What causes this and how can it be fixed?</p>