C1=ch_in C2=ch_out，第一个Conv是PW，第二个Conv是DW，且不需要激活函数

class SCDown(nn.Module):
    def __init__(self, c1, c2, k, s):
        super().__init__()
        self.cv1 = Conv(c1, c2, 1, 1)
        self.cv2 = Conv(c2, c2, k=k, s=s, g=c2, act=False)

    def forward(self, x):
        return self.cv2(self.cv1(x))