class Flatten:
    def __init__(self):
        pass
    def forward(self, inputs):
        self.C, self.W, self.H = inputs.shape
        return inputs.reshape(1, self.C*self.W*self.H)
    def backward(self, dy):
        return dy.reshape(self.C, self.W, self.H)
    def extract(self):
        return