Create a GAN from `learn_gen` and `learn_crit`.

def from_learners(cls, learn_gen:Learner, learn_crit:Learner, switcher:Callback=None,
                      weights_gen:Tuple[float,float]=None, **learn_kwargs):
        "Create a GAN from `learn_gen` and `learn_crit`."
        losses = gan_loss_from_func(learn_gen.loss_func, learn_crit.loss_func, weights_gen=weights_gen)
        return cls(learn_gen.data, learn_gen.model, learn_crit.model, *losses, switcher=switcher, **learn_kwargs)