def _setup_graph(self, input, get_cost_fn, get_opt_fn):
        """
        Implement the logic to build the graph, with an :class:`InputSource`
        that's been setup already.

        Returns:
            [Callback]: list of callbacks needed
        """

        Implement the logic to build the graph, with an :class:`InputSource`
        that's been setup already.

        Returns:
            [Callback]: list of callbacks needed
        
tensorpack/train/tower.py