[Pegasus] Refactor Tokenizer
Author: patrickvonplaten
Pull request: yes

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Fixes #8689, #8594, #8536

This PR refactors the Pegasus Tokenizer.

1st: It decouples the tokenizer from the Reformer Tokenizer because they don't really have much in common.
2nd: Pegasus' masked tokens are added. As stated in the [paper](https://arxiv.org/abs/1912.08777), PEGASUS has two masked tokens which are required for pre-training. Those two tokens `<mask_1>` and `<mask_2>` are added according to https://github.com/google-research/pegasus/blob/master/pegasus/ops/pretrain_parsing_ops.cc#L66 . This should solve or at least enable a solution for all three issues above.
3rd: IMO, all special tokens - which are in the case of Pegasus the tokens 2 to 104 - should be added to the `additional_special_tokens`. This is done here as well.

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 albert, bert, XLM: @LysandreJik
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 tokenizers: @mfuntowicz
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Comments:
Checked all slow and fast tests on GPU.

> You removed the links to google/sentencepiece but you kept the `Based on SentencePiece.`.
> 
> It seems to me that if we reference SentencePiece then it's good to keep a link to the library, no? Based on SentencePiece means that it's based on the library IMO, maybe you wanted to say it's based on Unigram instead?
> 
> Great changes, thanks for taking care of it!

Good point! I also think it would be nicer to have a link to it...For now, the text is always:

```
Construct a "fast" ALBERT tokenizer (backed by HuggingFace's `tokenizers` library). Based on SentencePiece.
```

=> So similar to what other FastTokenizers have written in their comments. But I agree that it could be confusing as "SentencePiece" doesn't really exist as an entity in `tokenizers` ... I think the "fast" sentencepiece tokenizers are either `BPE` or `Unigram` in tokenizers, no ? @thomwolf @n1t0 . Should I change the comments and link to their respective `tokenizers` model instead? So to 

```
Construct a "fast" ALBERT tokenizer (backed by HuggingFace's `tokenizers` library). Based on `Unigram <link to unigram in tokenizers>`__ . 
```

I think your proposal makes a lot of sense!