def __init__(self, towers, scale_gradient=True):
        """
        Args:
            towers(list[int]): list of GPU ids.
            scale_gradient (bool): if True, will scale each gradient by ``1.0/nr_gpu``.
        """
        super(AsyncMultiGPUBuilder, self).__init__(towers)
        self._scale_gradient = scale_gradient

        Args:
            towers(list[int]): list of GPU ids.
            scale_gradient (bool): if True, will scale each gradient by ``1.0/nr_gpu``.
        
tensorpack/graph_builder/training.py