Method backward

code/chapter9.py:1138–1144

(self, grad)

        return out, Xs
    
    def backward(self, grad):
        dXs = {}
        out = grad
        for k, v in reversed(list(self.layers.items())):
            dXs[k] = out
            out = v.backward(out)
        return out, dXs
    
    def update(self):
        """