Use save_safetensor to disable safe serialization for XLA (#28669)

* Use save_safetensor to disable safe serialization for XLA

https://github.com/huggingface/transformers/issues/28438

* Style fixup

src/transformers/trainer.py

@@ -2910,13 +2910,19 @@ class Trainer:
                     is_main_process=self.args.should_save,
                     state_dict=model.state_dict(),
                     save_function=xm.save,
+                    safe_serialization=self.args.save_safetensors,
                 )
             else:
                 logger.info("Trainer.model is not a `PreTrainedModel`, only saving its state dict.")
                 state_dict = model.state_dict()
                 xm.save(state_dict, os.path.join(output_dir, WEIGHTS_NAME))
         else:
-            model.save_pretrained(output_dir, is_main_process=self.args.should_save, save_function=xm.save)
+            model.save_pretrained(
+                output_dir,
+                is_main_process=self.args.should_save,
+                save_function=xm.save,
+                safe_serialization=self.args.save_safetensors,
+            )
         if self.tokenizer is not None and self.args.should_save:
             self.tokenizer.save_pretrained(output_dir)