def __init__(self, towers, ps_device):
        """
        Args:
            towers(list[int]): list of GPU id
            ps_device (str): either 'gpu' or 'cpu', where variables are stored.
        """
        super(SyncMultiGPUParameterServerBuilder, self).__init__(towers)
        assert ps_device in ['cpu', 'gpu']
        self.ps_device = ps_device

        Args:
            towers(list[int]): list of GPU id
            ps_device (str): either 'gpu' or 'cpu', where variables are stored.
        
tensorpack/graph_builder/training.py