title: KeyError: Frozen Tensorflow Model to UFF graph
body: <p>I have trained a custom CNN model with Tensorflow Estimator API. I have successfully Frozen the graph, but the conversion to UFF fails and throws following error:</p>
  
  <pre><code>'KeyError: u'IteratorGetNext:1'
  </code></pre>
  
  <p>The code to do the said conversion:</p>
  
  <pre><code>frozen_graph_filename = "Frozen_model.pb"
  TMP_UFF_FILENAME = "output.uff"
  output_name = "sigmoid"
  
  uff_model = uff.from_tensorflow_frozen_model(
      frozen_file=frozen_graph_filename,
      output_nodes=[output_name],
      output_filename=TMP_UFF_FILENAME,
      text=False,
  )
  </code></pre>
  
  <p>The names of the nodes in the graph are,</p>
  
  <pre><code>prefix/OneShotIterator
  prefix/IteratorGetNext
  prefix/Reshape/shape
  prefix/Reshape
  prefix/Reshape_1/shape
  prefix/Reshape_1
  prefix/conv1/kernel
  prefix/conv1/bias
  .
  .
  .
  prefix/logits/MatMul
  prefix/logits/BiasAdd
  prefix/sigmoid
  </code></pre>
  
  <p>So is there a way to remove the first two Iterator nodes? They are useless outside of the training context. I have also used <code>tf.graph_util.remove_training_nodes</code> but it does not alleviate the problem I am facing. </p>