[trainer] sharded _load_best_model (#17150)

* [trainer] sharded _load_best_model

probably needs a test?

* undo delete

src/transformers/trainer.py

@@ -1705,7 +1705,7 @@ class Trainer:
                 # If the model is on the GPU, it still works!
                 load_result = self.model.load_state_dict(state_dict, strict=False)
                 self._issue_warnings_after_load(load_result)
-        elif os.path.exists(best_model_path, os.path.join(self.state.best_model_checkpoint, WEIGHTS_INDEX_NAME)):
+        elif os.path.exists(os.path.join(self.state.best_model_checkpoint, WEIGHTS_INDEX_NAME)):
             # Best model is a sharded checkpoint
             load_result = load_sharded_checkpoint(self.model, self.state.best_model_checkpoint, strict=False)
             self._issue_warnings_after_load(load_result)