title: ValueError: not enough values to unpack (expected 3, got 2) - python/tensorflow
body: <p>I am trying to understand some code implemented using the tensorflow framework on jupyter notebook, when I get the following error on running this cell. I do not understand the problem here and some clarification would be very helpful.</p>
  
  <p>For reference, a training decoding layer is being created here in tensorflow.  </p>
  
  <p>Code: </p>
  
  <pre><code>   def decoding_layer_train(encoder_state, dec_cell, dec_embed_input, 
                           target_sequence_length, max_summary_length, 
                           output_layer, keep_prob):
      """
      Create a decoding layer for training
      :param encoder_state: Encoder State
      :param dec_cell: Decoder RNN Cell
      :param dec_embed_input: Decoder embedded input
      :param target_sequence_length: The lengths of each sequence in the target batch
      :param max_summary_length: The length of the longest sequence in the batch
      :param output_layer: Function to apply the output layer
      :param keep_prob: Dropout keep probability
      :return: BasicDecoderOutput containing training logits and sample_id
      """
      # TODO: Implement Function
  
      # Helper for the training process; used by Basic Decoder to read inputs
      training_helper = tf.contrib.seq2seq.TrainingHelper(inputs=dec_embed_input,
                                                          sequence_length=target_sequence_length,
                                                          time_major=False)
  
  
      # Basic decoder
      training_decoder = tf.contrib.seq2seq.BasicDecoder(dec_cell,
                                                         training_helper,
                                                         encoder_state,
                                                          output_layer)
  
  
      # Performs dynamic decoding using the decoder
      training_decoder_output, final_state, final_sequence_lengths = tf.contrib.seq2seq.dynamic_decode(training_decoder,
                                                                                                       impute_finished=True,
                                                                                                       maximum_iterations=max_summary_length)
  
      return training_decoder_output
  
  
  
  """
  DON'T MODIFY ANYTHING IN THIS CELL THAT IS BELOW THIS LINE
  """
  
  
  
  tests.test_decoding_layer_train(decoding_layer_train)enter code here
  </code></pre>
  
  <p>Error message (Screenshot):</p>
  
  <p><a href="https://i.stack.imgur.com/0gM2C.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/0gM2C.png" alt="Screenshot detailing the error"></a></p>