Fix `HubertRobustTest` PT/TF equivalence test on GPU
# What does this PR do?

Fix `HubertRobustTest` PT/TF equivalence test on GPU.

Note that `HubertRobustModelTest` has

```python
    def setUp(self):
        self.model_tester = HubertModelTester(
            self, conv_stride=(3, 3, 3), feat_extract_norm="layer", do_stable_layer_norm=True
        )
```

but `get_config()` had no ` do_stable_layer_norm=self.do_stable_layer_norm`

## To investigate further

- Why no issue on CPU even without this PR
- Why using `conv_stride=(4, 4, 4)` (the default value) has no issue on GPU, even without this PR

(Does this suggest we have PT/TF Hubert behave differently with `do_stable_layer_norm=False` on GPU when `conv_stride=(3, 3, 3)` etc?)

@patrickvonplaten You might have some idea about these points ..?

_The documentation is not available anymore as the PR was closed or merged._<|||||>Think it might be a great idea to add a check to avoid such situation occur in the future. Will do it in another PR.