[Bart] fix slow bart mask-infilling tests
Author: patrickvonplaten
Pull request: yes

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Fixes slow tests GPU Bart: https://github.com/huggingface/transformers/runs/3577103064?check_suite_focus=true#step:7:25556

```
tests/test_modeling_bart.py::BartModelIntegrationTests::test_base_mask_filling
tests/test_modeling_bart.py::BartModelIntegrationTests::test_large_mask_filling
```

cc @Narsil guess the output format has slightly changed? 

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Comments:
Seems odd that the output type has changed, it shouldn't have changed. The PR refactor was supossed to be with zero regression. 

I had no idea that there were tests of pipelines on GPU outside the test_pipelines_* location.



I think it's because we didn't have a test for single item list before, hence the backward incompatiblity, we need to override the `__call__` method of the `fill-mask` pipeline instead to behave oddly (as it was before)


Can we add other tests here too for GPU I think it's great to have performance checks.

Also the pipeline tests do not allow for GPU setting (anymore, and only some seem to have had the option), so that might explain it.

@Narsil - I don't follow here 100%. The tests were not just failing on GPU but also on CPU. What additional tests for pipelines should we add in your opinion exactly and where? 

IMO, we should **not** continue the practice of adding pipeline tests in model testing files such as `test_modeling_bart.py` (it's a very old test if I remember correctly). We could add more slow tests to the individual task specific tests, such as `test_pipelines_fill_mask.py`, but I think it'd be better to do this is a future PR.

Superseeded by : https://github.com/huggingface/transformers/pull/13540