Adding ciou and diou support in _box_loss()
          #5984
        
          
      
                
     Merged
            
            
          
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Related to #5983
Adding support of cIoU and dIoU loss on the private
_box_loss()used for regression. Currently this is an internal API used only by RetinaNet. I've done limited runs and seems that neither of the two losses improve performance:Hence I wont be making any changes on the
retinanet_resnet50_fpn_v2pretrained weights.