def __init__(self, towers):
        """
        Args:
            towers(list[int]): list of GPU ids.
        """
        if len(towers) > 1:
            logger.info("[DataParallel] Training a model of {} towers.".format(len(towers)))
            if not tf.test.is_built_with_cuda():
                logger.error("[DataParallel] TensorFlow was not built with CUDA support!")

        self.towers = towers

        Args:
            towers(list[int]): list of GPU ids.
        
tensorpack/graph_builder/training.py