Adding `skip_special_tokens=True` to FillMaskPipeline
Author: Narsil
Pull request: yes

# What does this PR do?

- It's backward incompatible.
- It makes for sense for pipelines to remove references to special_tokens
(all of the other pipelines do that).
- Keeping special tokens makes it hard for users to actually remove them
because all models have different tokens (`
`, `<cls>`,` [CLS]`, ....)
- It's actually closer to the docs specs : 
```
            - **sequence** (:obj:`str`) -- The corresponding input with the mask token prediction.
```
as the input does not include the special tokens.

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Linked to : https://github.com/huggingface/transformers/issues/9518



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Comments:
If it's a bug users relied on, then it's a feature, not a bug. 

Anyway I'll merge this.

Good point, but I don't think that's the case here. Do you think users rely on the previous behavior? If that's the case, then we should revert this PR until we change of major version.