def start_bundle(self, element=None):
    """Build the transfromation graph once."""
    import tensorflow as tf
    from trainer import feature_transforms

    g = tf.Graph()
    session = tf.Session(graph=g)

    # Build the transformation graph
    with g.as_default():
      transformed_features, _, placeholders = (
          feature_transforms.build_csv_serving_tensors_for_transform_step(
              analysis_path=self._analysis_output_dir, 
              features=self._features, 
              schema=self._schema,
              stats=self._stats,
              keep_target=True))
      session.run(tf.tables_initializer())
    
    self._session = session
    self._transformed_features = transformed_features
    self._input_placeholder_tensor = placeholders['csv_example']
Build the transfromation graph once.