title: AttributeError: 'list' object has no attribute 'shape'
body: <p>I am following the demo code on Tensorflow CNN's, and I've ran into this error involving logging_hook</p>
  
  <blockquote>
    <p>File "Machine_Learning/CNN_model_creator.py", line 105, in main
        hooks=[logging_hook]</p>
  </blockquote>
  
  <p>Here is a snippet of my code:</p>
  
  <pre><code>    #create the estimator
  clf = tf.estimator.Estimator(model_fn=cnn_model_fn, model_dir="/model")
  
  #set up logging for predictions
  #log the values in the "softmax" tensor with label "probabilities"
  tensors_to_log = {"probabilities":"softmax_tensor"}
  logging_hook = tf.train.LoggingTensorHook(tensors=tensors_to_log, every_n_iter=50)
  
  #train the model_f
  train_input_fn = tf.estimator.inputs.numpy_input_fn(
      x={"x": train_data},
      y=train_labels,
      batch_size=100,
      num_epochs=None,
      shuffle=True
  )
  clf.train(
      input_fn=train_input_fn,
      steps=20000,
      hooks=[logging_hook]
  )
  </code></pre>
  
  <p>I don't fully understand the purpose of logging_hook, or why it needs to be in a list when its value is passed to hooks. Any help is appreciated.</p>