Adding `Perceiver` to `AutoTokenizer`.
Author: Narsil
Pull request: yes

# What does this PR do?

Make

```python
tokenizer = AutoTokenizer.from_pretrained("deepmind/language-perceiver")
```
work.

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Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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