It looks like in 13 some things about what could be compiled in FunctionLayer changed.

In 12.3, I had to write some ugly code to get the anchor box to box output conversion network to compile in a FunctionLayer, so before it looked like this:

FunctionLayer[Apply[Function[{anchorsIn, grid, conv}, Block[
(* This is a really bad function because in 12.2, FunctionLayer was not compiling nice functions. *)
(* ... *)
(* We assume the net has been reshaped to dimensions n x Anchors x dim x dim.  *)
{
  boxes = conv[[1 ;; 4]],
  classPredictions = conv[[6 ;;]],
  confidences = LogisticSigmoid[conv[[5]]]
},
(* We first need to construct our boxes to find the best fits. *)

Block[{
  boxesScaled =
      Join[((1.0/(inputSize/32.0)*Tanh[boxes[[1 ;; 2]]] // TransposeLayer[{2 <-> 3, 3 <-> 4}]) +
          (grid // TransposeLayer[{1 <-> 3, 3 <-> 2}]) // TransposeLayer[{4 <-> 2, 3 <-> 4}]),
        (Transpose @ (5*LogisticSigmoid[ boxes[[2 ;; 3]]]) * anchorsIn // Transpose)]},
  <|
    "boxes" -> boxesScaled,
    "confidences" -> confidences,
    "classes" -> classPredictions
  |>
]]]]]

In 13, it seems like compiling nested blocks/modules/whatever does not work anymore, but we can simplify this expression to a single module now without getting compilation errors.

FunctionLayer[Apply[Function[{anchorsIn, grid, conv}, Module[
    (* We assume the net has been reshaped to dimensions n x Anchors x dim x dim.  *)
    {
      boxes = conv[[1 ;; 4]],
      classPredictions = conv[[6 ;;]],
      confidences = LogisticSigmoid[conv[[5]]]
    },
    (* We first need to construct our boxes to find the best fits. *)
      <|
        "boxes" -> Join[((1.0/(inputSize/32.0)*Tanh[boxes[[1 ;; 2]]] // TransposeLayer[{2 <-> 3, 3 <-> 4}]) +
              (grid // TransposeLayer[{1 <-> 3, 3 <-> 2}]) // TransposeLayer[{4 <-> 2, 3 <-> 4}]),
            (Transpose @ (5*LogisticSigmoid[ boxes[[2 ;; 3]]]) * anchorsIn // Transpose)],
        "confidences" -> confidences,
        "classes" -> classPredictions
      |>
    ]]]]

Lastly, it seems the convention for calling ThreadingLayer and FunctionLayer with multiple arguments changed:

before, you could do

ThreadingLaye[...][x, y]

but in 13 you need to do:

ThreadingLayer[...][{x, y}]

And now it works again:

I have pushed these changes to the Wolf Detector github repo, so older versions are probably broken now. But it now works in 13.2. It is also worth noting that 13 added the built-in function TrainImageContentDetector, but IDK what algorithm it is using. Maybe someone from WRI can chime in on that?

Dear Alec, I am currently working on a wood defects detector for my bachelor graduation thesis, and I am trying to implement this using Yolov2.
I ran through your packages and your code you did great job writing them and I would like to mention that they are the only helpful resources i found on the internet for training an object detector on custom data using Mathematica.
However, I tried your Packages and each time I run the BuildYoloLoss package I get the following error FunctionLayer::compilerr: Cannot interpret ThreadingLayer[<>][#1, #2] & as a network. I am running Version 13.01, and since you wrote your code on earlier versions, I think there is an issue related to the syntax of the function layers specification in the GIOU Loss package. I would appreciate your help on that, and if you have other resources or insights on how to build similar loss functions and perform transfer learning on recent Yolo versions using Mathematica, as I cant find any. Thanks in advance.