Writes training metrics to Tensorboard.

def _write_metrics(self, iteration:int, last_metrics:MetricsList, start_idx:int=2)->None:
        "Writes training metrics to Tensorboard."
        recorder = self.learn.recorder
        for i, name in enumerate(recorder.names[start_idx:]):
            if last_metrics is None or len(last_metrics) < i+1: return
            scalar_value = last_metrics[i]
            self._write_scalar(name=name, scalar_value=scalar_value, iteration=iteration)