text: Title: Only 2 feature map size shrinkages in extra_layer
  Body: Hi, I print the layers in extra_layer.  I think there should be 4 feature map shrinkages according to the paper, but found only 2 feature map shrinkages. 
  ```bash
  [Conv2d(1024, 256, kernel_size=(1, 1), stride=(1, 1)),                                         
   Conv2d(256, 512, kernel_size=(3, 3), stride=(2, 2), padding=(1, 1)),                           
   Conv2d(512, 128, kernel_size=(1, 1), stride=(1, 1)),                                            
   Conv2d(128, 256, kernel_size=(3, 3), stride=(2, 2), padding=(1, 1)),                             
   Conv2d(256, 128, kernel_size=(1, 1), stride=(1, 1)),
   Conv2d(128, 256, kernel_size=(3, 3), stride=(1, 1)), ◀
   Conv2d(256, 128, kernel_size=(1, 1), stride=(1, 1)),
   Conv2d(128, 256, kernel_size=(3, 3), stride=(1, 1)) ◀]
  ```