Callback function that writes backward end appropriate data to Tensorboard.

def on_backward_end(self, iteration:int, **kwargs)->None:
        "Callback function that writes backward end appropriate data to Tensorboard."
        if iteration == 0: return
        self._update_batches_if_needed()
        if iteration % self.stats_iters == 0: self._write_model_stats(iteration=iteration)