Method update

code/chapter9.py:1146–1152

函数作用：梯度更新

(self)

        return out, dXs
    
    def update(self):
        """
        函数作用：梯度更新
        """
        for k, v in reversed(list(self.layers.items())):
            v.update()
        self.flush_gradients()
    
    def flush_gradients(self, curr_loss=None):
        """