Add support for metadata format MLX (#29335)

Add support for loading safetensors files saved with metadata format mlx.

src/transformers/modeling_utils.py

@@ -504,7 +504,7 @@ def load_state_dict(checkpoint_file: Union[str, os.PathLike]):
         # Check format of the archive
         with safe_open(checkpoint_file, framework="pt") as f:
             metadata = f.metadata()
-        if metadata.get("format") not in ["pt", "tf", "flax"]:
+        if metadata.get("format") not in ["pt", "tf", "flax", "mlx"]:
             raise OSError(
                 f"The safetensors archive passed at {checkpoint_file} does not contain the valid metadata. Make sure "
                 "you save your model with the `save_pretrained` method."