Change os.path to pathlib Path (#6737)

Change os.path to pathlib

src/diffusers/pipelines/onnx_utils.py

@@ -172,7 +172,7 @@ class OnnxRuntimeModel:
         # load model from local directory
         if os.path.isdir(model_id):
             model = OnnxRuntimeModel.load_model(
-                os.path.join(model_id, model_file_name), provider=provider, sess_options=sess_options
+                Path(model_id, model_file_name).as_posix(), provider=provider, sess_options=sess_options
             )
             kwargs["model_save_dir"] = Path(model_id)
         # load model from hub