title: tuple index out of range in tensorflow Concatenate
body: <p>This is my first time using concatenate,
  I am building a NN for a deep learning project,
  one class defines the concatenate, and that class runs fine.</p>
  <p>The next def includes concatenate, and it is giving me a ' tuple index out of range&quot;
  I am fairly confused of where to fix this..
  I m uploading parts of my code, please help.</p>
  <pre><code>def call(self, x):
      self.conc = concatenate([])
      x = self.flatten(x)
      r = x # residual / short-cut / skip connection
      if self.probe_num == -1: # for network training
        for (i, layer) in enumerate(self.my_layers):
          if i == SkipLayer_2:
            x = self.conc([x, r])
            x = layer(x)
            return x
      else: # for probe training
        for (i, layer) in enumerate(self.my_layers[0:self.probe_num]):
          if i == SkipLayer_2:
            x = self.conc([x, r])
            x = layer(x)
            x = tf.stop_gradient(x)
            probe = self.probes[self.probe_num]
      return probe(x)
  </code></pre>
  <p>and</p>
  <pre><code>for probe_num in range(len(model.my_probe)):
    model.probe_num = probe_num
    model(X_train[0:Batch_2])
  weights = model.get_weights()
  
  probe_error = probe_training(weights)
  </code></pre>
  <p>give me</p>
  <pre><code>   IndexError                                Traceback (most recent call last)
  &lt;ipython-input-22-bfb6ffd6d257&gt; in &lt;module&gt;()
       51 for probe_num in range(len(model.my_probe)):
       52   model.probe_num = probe_num
  ---&gt; 53   model(X_train[0:Batch_2])
       54 weights = model.get_weights()
       55 
  
  
  &lt;ipython-input-21-6f2bfe4c9db1&gt; in call(self, x)
       21     # Call
       22   def call(self, x):
  ---&gt; 23     self.conc = concatenate([])
       24     x = self.flatten(x)
       25     r = x # residual / short-cut / skip connection
  
  /usr/local/lib/python3.6/dist-packages/tensorflow/python/keras/layers/merge.py in concatenate(inputs, axis, **kwargs)
      929       A tensor, the concatenation of the inputs alongside axis `axis`.
      930   &quot;&quot;&quot;
  --&gt; 931   return Concatenate(axis=axis, **kwargs)(inputs)
  
  
  7 frames
  /usr/local/lib/python3.6/dist-packages/tensorflow/python/keras/layers/merge.py in build(self, input_shape)
      490   def build(self, input_shape):
      491     # Used purely for shape validation.
  --&gt; 492     if not isinstance(input_shape[0], tuple) or len(input_shape) &lt; 2:
      493       raise ValueError('A `Concatenate` layer should be called '
      494                        'on a list of at least 2 inputs')
  
  IndexError: tuple index out of range
  </code></pre>