Writes gradient statistics for generator to Tensorboard.

def _write_gen_model_stats(self, iteration:int)->None:
        "Writes gradient statistics for generator to Tensorboard."
        generator = self.learn.gan_trainer.generator
        self.stats_writer.write(model=generator, iteration=iteration, tbwriter=self.tbwriter, name='gen_model_stats')
        self.gen_stats_updated = True