How to change PyTorch faster-rcnn losses?

**Hi experts,
> *
*** I try to make transfer learning on fasterrcnn_resnet50_fpn pretrained model. My dataset is highly imbalanced (323,3332,127 samples for 3 classes). I think I can overcome this problem with manipulation of the classifier loss. But I couldn’t find a way to change the NLL loss to focal loss. How can I manipulate the losses of the pretrained model.