Failing ONNX Slow Test
```
___________________ OnnxExportTestCase.test_quantize_pytorch ___________________
[gw0] linux -- Python 3.7.6 /home/hf/actions-runner_transformers/_work/transformers/transformers/.env/bin/python

self = <tests.test_onnx.OnnxExportTestCase testMethod=test_quantize_pytorch>

    @require_torch
    @slow
    def test_quantize_pytorch(self):
        for model in OnnxExportTestCase.MODEL_TO_TEST:
            path = self._test_export(model, "pt", 12)
            quantized_path = quantize(path)
    
            # Ensure the actual quantized model is not bigger than the original one
>           if quantized_path.stat().st_size >= Path(path).stat().st_size:
E           AttributeError: 'NoneType' object has no attribute 'stat'

tests/test_onnx.py:76: AttributeError
=============================== warnings summary ===============================

```

https://github.com/huggingface/transformers/runs/996031882?check_suite_focus=true
@sshleifer Is it still a failing test? I can have a look<|||||>Still failing! See https://github.com/huggingface/transformers/runs/1024137280?check_suite_focus=true<|||||>On it 💪 <|||||>attempt at: #6716