Where can I find how to deal with the return of the forward method?

edit

There are code examples using SDD, Mask-RCNN and YOLOv3 on the internet but they didn't explain the logic behind the list or array manipulation that the forward method returns.

Now, I'm trying to execute Faster-RCNN model I downloaded from this repo. The problem is that none of the logic used in the other methods are going well.