Alright, so the issue was indeed that the forward method of one of the underlying nn.Module objects was not working, my bad.
Thanks for the advice ayalaa2
If anyone has inputs on the other part of the question (is there an easy way to debug this kind of thing, or at least a hard way, but a hard way that can systematically point me to the exact line where the flow stops), I’d still love to hear it, as I’ll be facing these kinds of issues a lot in the future.