# Codeblock 13a
class YOLOv3(nn.Module):
    def __init__(self):
        super().__init__()
        
        ###############################################
        # Backbone initialization.
        
        self.darknet53 = Darknet53()    #(1)
        
        
        ###############################################
        # For 13x13 output.
        
        self.conv0  = Convolutional(in_channels=1024, out_channels=512, kernel_size=1)
        self.conv1  = Convolutional(in_channels=512, out_channels=1024, kernel_size=3)
        self.conv2  = Convolutional(in_channels=1024, out_channels=512, kernel_size=1)
        self.conv3  = Convolutional(in_channels=512, out_channels=1024, kernel_size=3)
        self.conv4  = Convolutional(in_channels=1024, out_channels=512, kernel_size=1)
        
        self.detection_head_large_obj = DetectionHead(num_channels=512)
        
        
        ###############################################
        # For 26x26 output.
        
        self.conv5  = Convolutional(in_channels=512, out_channels=256, kernel_size=1)  #(2)
        self.upsample0 = nn.Upsample(scale_factor=2)      #(3)
        
        self.conv6  = Convolutional(in_channels=768, out_channels=256, kernel_size=1)
        self.conv7  = Convolutional(in_channels=256, out_channels=512, kernel_size=3)
        self.conv8  = Convolutional(in_channels=512, out_channels=256, kernel_size=1)
        self.conv9  = Convolutional(in_channels=256, out_channels=512, kernel_size=3)
        self.conv10 = Convolutional(in_channels=512, out_channels=256, kernel_size=1)
        
        self.detection_head_medium_obj = DetectionHead(num_channels=256)
        
        
        ###############################################
        # For 52x52 output.
        
        self.conv11  = Convolutional(in_channels=256, out_channels=128, kernel_size=1)  #(4)
        self.upsample1 = nn.Upsample(scale_factor=2)      #(5)
        
        self.conv12  = Convolutional(in_channels=384, out_channels=128, kernel_size=1)
        self.conv13  = Convolutional(in_channels=128, out_channels=256, kernel_size=3)
        self.conv14  = Convolutional(in_channels=256, out_channels=128, kernel_size=1)
        self.conv15  = Convolutional(in_channels=128, out_channels=256, kernel_size=3)
        self.conv16  = Convolutional(in_channels=256, out_channels=128, kernel_size=1)
        
        self.detection_head_small_obj = DetectionHead(num_channels=128)