Switch the model if necessary.

def on_batch_end(self, iteration, **kwargs):
        "Switch the model if necessary."
        if self.learn.gan_trainer.gen_mode:
            self.n_g += 1
            n_iter,n_in,n_out = self.n_gen,self.n_c,self.n_g
        else:
            self.n_c += 1
            n_iter,n_in,n_out = self.n_crit,self.n_g,self.n_c
        target = n_iter if isinstance(n_iter, int) else n_iter(n_in)
        if target == n_out:
            self.learn.gan_trainer.switch()
            self.n_c,self.n_g = 0,0