Writes training loss to Tensorboard.

def _write_training_loss(self, iteration:int, last_loss:Tensor)->None:
        "Writes training loss to Tensorboard."
        recorder = self.learn.gan_trainer.recorder
        if len(recorder.losses) == 0: return
        scalar_value = to_np((recorder.losses[-1:])[0])
        tag = self.metrics_root + 'train_loss'
        self.tbwriter.add_scalar(tag=tag, scalar_value=scalar_value, global_step=iteration)