title: ValueError: Operation u'tpu_140462710602256/VarIsInitializedOp' has been marked as not fetchable
body: <p>The code works fine on GPU and CPU.But when I use keras_to_tpu_model function to make the model able to run on TPU, the error occurred.     </p>
  
  <p>This is the full output on colab:<a href="https://colab.research.google.com/gist/WangHexie/2252beb26f16354cb6e9ba2639970e5b/tpu-error.ipynb" rel="nofollow noreferrer">https://colab.research.google.com/gist/WangHexie/2252beb26f16354cb6e9ba2639970e5b/tpu-error.ipynb</a><br>
  Change runtype to TPU,I think this can be reproduced.         </p>
  
  <p>Code on github:<a href="https://github.com/WangHexie/DHNE/blob/master/src/hypergraph_embedding.py#L60" rel="nofollow noreferrer">https://github.com/WangHexie/DHNE/blob/master/src/hypergraph_embedding.py#L60</a>     </p>
  
  <p>You can test the code on GPU by changing to the gpu branch.           </p>
  
  <p>Traceback</p>
  
  <pre><code>    Traceback (most recent call last):
    File "src/hypergraph_embedding.py", line 158, in &lt;module&gt;
      h.train(dataset)
    File "src/hypergraph_embedding.py", line 75, in train
      epochs=self.options.epochs_to_train, verbose=1)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/keras/engine/training.py", line 2177, in fit_generator
      initial_epoch=initial_epoch)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/keras/engine/training_generator.py", line 176, in fit_generator
      x, y, sample_weight=sample_weight, class_weight=class_weight)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/keras/engine/training.py", line 1940, in train_on_batch
      outputs = self.train_function(ins)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/contrib/tpu/python/tpu/keras_support.py", line 1238, in __call__
      infeed_manager)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/contrib/tpu/python/tpu/keras_support.py", line 1143, in _tpu_model_ops_for_input_specs
      infeed_manager)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/contrib/tpu/python/tpu/keras_support.py", line 1053, in _specialize_model
      _model_fn, inputs=[[]] * self._tpu_assignment.num_towers)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/contrib/tpu/python/tpu/tpu.py", line 687, in split_compile_and_replicate
      outputs = computation(*computation_inputs)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/contrib/tpu/python/tpu/keras_support.py", line 959, in _model_fn
      self.model.cpu_optimizer)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/contrib/tpu/python/tpu/keras_support.py", line 378, in _clone_optimizer
      config = optimizer.get_config()
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/keras/optimizers.py", line 275, in get_config
      'lr': float(K.get_value(self.lr)),
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/keras/backend.py", line 2709, in get_value
      return x.eval(session=get_session())
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/keras/backend.py", line 469, in get_session
      _initialize_variables(session)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/keras/backend.py", line 731, in _initialize_variables
      [variables_module.is_variable_initialized(v) for v in candidate_vars])
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/client/session.py", line 929, in run
      run_metadata_ptr)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/client/session.py", line 1137, in _run
      self._graph, fetches, feed_dict_tensor, feed_handles=feed_handles)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/client/session.py", line 484, in __init__
      self._assert_fetchable(graph, fetch.op)
    File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/client/session.py", line 497, in _assert_fetchable
      'Operation %r has been marked as not fetchable.' % op.name)
  ValueError: Operation u'tpu_140276544043536/VarIsInitializedOp' has been marked as not fetchable.
  </code></pre>