Added scale_x_y param to [yolo]-layer (for sigmoid).

In previous commit is used GIoU(generalized-iou) from  https://github.com/generalized-iou/g-darknet

include/darknet.h

@@ -314,6 +314,7 @@ struct layer {
     float *weights;
     float *weight_updates;
 
+    float scale_x_y;
     float iou_normalizer;
     float cls_normalizer;
     int iou_loss;