def fill_feed_dict(data_set, images_pl, labels_pl):
    """Fills the feed_dict for training the given step.
    
    A feed_dict takes the form of:
    feed_dict = {
        <placeholder>: <tensor of values to be passed for placeholder>,
        ....
    }
    
    Args:
      data_set: The set of images and labels, from input_data.read_data_sets()
      images_pl: The images placeholder, from placeholder_inputs().
      labels_pl: The labels placeholder, from placeholder_inputs().
    
    Returns:
      feed_dict: The feed dictionary mapping from placeholders to values.
    """
    # Create the feed_dict for the placeholders filled with the next
    # `batch size ` examples.
    images_feed, labels_feed = data_set._next()
    feed_dict = {
        images_pl: images_feed,
        labels_pl: labels_feed,
    }
    return feed_dict