title: NoneType has no attribute 'select' KerasDML SystemML
body: <p>Im have some issue while running example Keras2DML code in <a href="http://apache.github.io/systemml/beginners-guide-keras2dml#model-conversion" rel="nofollow noreferrer">this page</a>. While running the code, i've got this error: </p>
  
  <pre><code>Traceback (most recent call last):
    File "/home/fregy/kerasplayground/sysml/examplenn.py", line 12, in &lt;module&gt;
      sysml_model = Keras2DML(spark, keras_model,input_shape=(3,224,224))
    File "/usr/local/lib/python2.7/dist-packages/systemml/mllearn/estimators.py", line 909, in __init__
      convertKerasToCaffeNetwork(keras_model, self.name + ".proto")
    File "/usr/local/lib/python2.7/dist-packages/systemml/mllearn/keras2caffe.py", line 201, in convertKerasToCaffeNetwork
      jsonLayers = list(chain.from_iterable(imap(lambda layer: _parseKerasLayer(layer), kerasModel.layers)))
    File "/usr/local/lib/python2.7/dist-packages/systemml/mllearn/keras2caffe.py", line 201, in &lt;lambda&gt;
      jsonLayers = list(chain.from_iterable(imap(lambda layer: _parseKerasLayer(layer), kerasModel.layers)))
    File "/usr/local/lib/python2.7/dist-packages/systemml/mllearn/keras2caffe.py", line 137, in _parseKerasLayer
      ret = { 'layer': { 'name':layer.name, 'type':supportedLayers[layerType], 'bottom':_getBottomLayers(layer), 'top':layer.name, paramName:param[paramName] } }
    File "/usr/local/lib/python2.7/dist-packages/systemml/mllearn/keras2caffe.py", line 112, in _getBottomLayers
      return [ bottomLayer.name for bottomLayer in _getInboundLayers(layer) ]
    File "/usr/local/lib/python2.7/dist-packages/systemml/mllearn/keras2caffe.py", line 70, in _getInboundLayers
      for node in layer.inbound_nodes:  # get inbound nodes to current layer
  AttributeError: 'Conv2D' object has no attribute 'inbound_nodes'
  Exception in thread Thread-1:
  Traceback (most recent call last):
    File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
      self.run()
    File "/usr/lib/python2.7/threading.py", line 754, in run
      self.__target(*self.__args, **self.__kwargs)
    File "/usr/lib/python2.7/SocketServer.py", line 230, in serve_forever
      r, w, e = _eintr_retry(select.select, [self], [], [],
  AttributeError: 'NoneType' object has no attribute 'select'
  </code></pre>
  
  <p>Im using Tensorflow-GPU 1.5 , and Keras 2.1.3 .</p>