# Codeblock 9
class DetectionModel(nn.Module):
    def __init__(self):
        super().__init__()
        
        self.cnn = CNN()
        self.fpn = FPN()
        self.rpn = RPN()
        
    def forward(self, x):
        
        c2, c3, c4, c5 = self.cnn(x)                 #(1)
        p2, p3, p4, p5 = self.fpn(c2, c3, c4, c5)    #(2)
        
        p2_pred = self.rpn(p2)        #(3)
        p3_pred = self.rpn(p3)
        p4_pred = self.rpn(p4)
        p5_pred = self.rpn(p5)        #(4)
        
        return p2_pred, p3_pred, p4_pred, p5_pred