I am not sure what is your programming style in Pytorch. As people recommended and in most tutorials I see the sequential approach, where a small mistake of data preprocessing would lead to clueless errors in a completely irrelevant line.

YOLO is a quite standard feed-forward model in my opinion. I mean the math part, which I am more concerned with.

I have never used Theano before, my idea from it is that Tensorflow followed its static graph approach.