src/transformers/trainer.py/Trainer/_save_tpu
class Trainer:    def _save_tpu(self, output_dir: Optional[str] = None):
        output_dir = output_dir if output_dir is not None else self.args.output_dir
        logger.info(f"Saving model checkpoint to {output_dir}")

        if xm.is_master_ordinal():
            os.makedirs(output_dir, exist_ok=True)
            torch.save(self.args, os.path.join(output_dir, TRAINING_ARGS_NAME))

        # Save a trained model and configuration using `save_pretrained()`.
        # They can then be reloaded using `from_pretrained()`
        xm.rendezvous("saving_checkpoint")
        if not isinstance(self.model, PreTrainedModel):
            if isinstance(unwrap_model(self.model), PreTrainedModel):
                unwrap_model(self.model).save_pretrained(
                    output_dir,
                    is_main_process=self.args.should_save,
                    state_dict=self.model.state_dict(),
                    save_function=xm.save,
                )
            else:
                logger.info("Trainer.model is not a `PreTrainedModel`, only saving its state dict.")
                state_dict = self.model.state_dict()
                xm.save(state_dict, os.path.join(output_dir, WEIGHTS_NAME))
        else:
            self.model.save_pretrained(output_dir, is_main_process=self.args.should_save, save_function=xm.save)
        if self.tokenizer is not None and self.args.should_save:
            self.tokenizer.save_pretrained(output_dir)