Writes gradient statistics for critic to Tensorboard.

def _write_critic_model_stats(self, iteration:int)->None:
        "Writes gradient statistics for critic to Tensorboard."
        critic = self.learn.gan_trainer.critic
        self.stats_writer.write(model=critic, iteration=iteration, tbwriter=self.tbwriter, name='crit_model_stats')
        self.crit_stats_updated = True