>An iterable containing all feature columns. All items
should be instances of classes derived from <code>_FeatureColumn</code>.</p></td>
    </tr>
    <tr>
      <th>label_key</th>
      <td><p>A string identifying the label. It means <code>tf$Example</code> stores
labels with this key.</p></td>
    </tr>
    <tr>
      <th>label_dtype</th>
      <td><p>A <code>tf$dtype</code> identifies the type of labels. By default it
is <code>tf$float32</code>.</p></td>
    </tr>
    <tr>
      <th>label_default</th>
  