broken ONNX slow test
```
    def test_quantize_pytorch(self):
        for model in OnnxExportTestCase.MODEL_TO_TEST:
            path = self._test_export(model, "pt", 12)
>           quantized_path = quantize(Path(path))
```
tests/test_onnx.py:75:  `path` is None

https://github.com/huggingface/transformers/runs/960368281?check_suite_focus=true
This seems to be a bit of a flaky test, doesn't it?<|||||>There is strange try/except syntax in `_test_export ` that I think can be trivially improved.