def backward(self, x, gy):
        xp = cuda.get_array_module(*x)

        gx = xp.empty_like(x[0])
        if self.axis is None:
            gx[:] = gy[0]
        else:
            gy = gy[0]
            actual_axis = []
            for axis in self.axis:
                if 0 < axis:
                    axis = len(gx.shape) + axis
                actual_axis.append(axis)
            for axis in sorted(actual_axis):
                gy = xp.expand_dims(gy, axis=axis)
            gx[:] = gy

        return gx,
dataset/ETHPy150Open pfnet/chainer/chainer/functions/math/sum.py/Sum.backward/swapped operands of `<`