Include safetensors as part of `_load_best_model` (#30553)

* Include safetensors

* Cleanup

src/transformers/trainer.py

@@ -2611,7 +2611,9 @@ class Trainer:
                     load_result = model.load_state_dict(state_dict, False)
                 if not is_sagemaker_mp_enabled() and has_been_loaded:
                     self._issue_warnings_after_load(load_result)
-        elif os.path.exists(os.path.join(self.state.best_model_checkpoint, WEIGHTS_INDEX_NAME)):
+        elif os.path.exists(os.path.join(self.state.best_model_checkpoint, SAFE_WEIGHTS_INDEX_NAME)) or os.path.exists(
+            os.path.join(self.state.best_model_checkpoint, WEIGHTS_INDEX_NAME)
+        ):
             load_result = load_sharded_checkpoint(
                 model, self.state.best_model_checkpoint, strict=is_sagemaker_mp_enabled()
             )