Switch the model, if `gen_mode` is provided, in the desired mode.

def switch(self, gen_mode:bool=None):
        "Switch the model, if `gen_mode` is provided, in the desired mode."
        self.gen_mode = (not self.gen_mode) if gen_mode is None else gen_mode
        self.opt.opt = self.opt_gen.opt if self.gen_mode else self.opt_critic.opt
        self._set_trainable()
        self.model.switch(gen_mode)
        self.loss_func.switch(gen_mode)