Can any classifier be used as the critic for GANLearner? (Lesson 7)

Hi! I’ve been playing with the GAN example from Lesson 7 and I struggle to understand how the critic that we create works. It is supposed to be a binary classifier, but it returns more than 2 numbers; it also seems quite different from a classifier created by calling cnn_learner(). Following from that, another thing I would appreciate some insight with is whether any classifier (i.e. one created with cnn_learner()) could be used by GANLearner as its critic? So far my attempts to do so have been unsuccessful.

Thanks for any help in advance.