title: "None" dimension causes error when using DataSet API Tensorflow
body: <p>
  I am trying to use Dataset API to feed the resnet found in the latest <a href="https://github.com/tensorflow/models/tree/master/official" rel="nofollow noreferrer">Tensorflow official models release</a>.</p>
  
  <p>The basic code is as follows:</p>
  
  <pre class="lang-py prettyprint-override"><code>with tf.Session() as sess:
      print("initialized")
  
      features_placeholder = tf.placeholder(prepared_x.dtype, prepared_x.shape)
      labels_placeholder = tf.placeholder(dtype=tf.float32, shape=prepared_t.shape)
  
      dataset = tf.contrib.data.Dataset.from_tensor_slices((features_placeholder, labels_placeholder))
      dataset = dataset.shuffle(buffer_size=10000)
      dataset = dataset.batch(batch_size)
      dataset = dataset.repeat(num_epoch)
  
      iterator = dataset.make_initializable_iterator()
  
      (next_x_test, next_t_test) = iterator.get_next()
      next_x_test = tf.to_float(next_x_test, name='ToFloat')
  
  
      sess.run(iterator.initializer, feed_dict={features_placeholder: prepared_x,
                                                labels_placeholder: prepared_t})
  
  
      print(next_x_test)
      print(next_t_test)
  
      model = resnet_v2(resnet_size=50, num_classes=num_bins)
  
      output = model(next_x_test,is_training=True)
  </code></pre>
  
  <p>This last lines throws an error when compiling </p>
  
  <blockquote>
    <p>ValueError: The last dimension of the inputs to <code>Dense</code> should be
    defined. Found <code>None</code>.</p>
  </blockquote>
  
  <p>which makes reference back to the <code>resent_v2</code> definition where the final layer is a dense layer.  </p>
  
  <p>How can I assert the shape of my features tensor?</p>