Adding Llama FastTokenizer support.
- Requires https://github.com/huggingface/tokenizers/pull/1183 version
- Only support byte_fallback for llama, raise otherwise (safety net).
- Lots of questions are special tokens

How to test:

```python
#! pip install -e https://github.com/huggingface/tokenizers@byte_fallback#egg=tokenizers

from transformers.convert_slow_tokenizer import convert_slow_tokenizer
from transformers import AutoTokenizer
from tokenizers import Tokenizer

tokenizer = AutoTokenizer.from_pretrained("huggingface/llama-7b")

if False:
    new_tokenizer = Tokenizer.from_file("tok.json")
else:
    new_tokenizer = convert_slow_tokenizer(tokenizer)
    new_tokenizer.save("tok.json")

strings = [
    "This is a test",
    "生活的真谛是",
    "生活的真谛是[MASK]。",
    # XXX: This one is problematic because of special tokens
    # "<s> Something something",
]

for string in strings:
    encoded = tokenizer(string)["input_ids"]
    encoded2 = new_tokenizer.encode(string).ids

    assert encoded == encoded2, f"{encoded} != {encoded2}"

    decoded = tokenizer.decode(encoded)
    decoded2 = new_tokenizer.decode(encoded2)

    assert decoded.strip() == decoded2, f"{repr(decoded)} != {repr(decoded2)}"
```


# What does this PR do?

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Models:

- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts
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Library:

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 -->
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for the ping. We'll need the actual fast tokenizer file to merge this though :sweat_smile: <|||||>True, I uncovered more issues around multiple space handling, I'm nailing down on the pre_tokenizer combo for it.<|||||>More troublesome than anticipated.

When encoding `" Hello"` from a pure BPE perspectivve, `tokenizers` does `[259, 10994]`  (`"  "` + `Hello`)
whereas spm does `[29871, 15043]` (`" "` +  `" Hello"`) which from a pure ids & merges perspectives seems worse.

I though of fixing that using a `pre_tokenizer` that splits words onto their own.

However on encoding `"  ird"` this time `spm` DOES do `[259, 1823]`.
Seems this is where the score comes into play.<|||||>What is the status of this PR?
<|||||>For the doc builder, we're going to need an update on the docker image so that it pulls 0.13.3 to generate the doc.<|||||>Hi @Narsil ,

the `warning.warn` to `raise RuntimeError` change in `src/transformers/convert_slow_tokenizer.py` breaks a lot of things: I wanted to fine-tune a mT5 model and it is now no longer possible (I'm using the PyTorch example from [documentation](https://github.com/huggingface/transformers/tree/main/examples/pytorch/question-answering#fine-tuning-t5-on-squad20).)

How is it possible to rubustify it -> also DeBERTa v3 has byte fallback vocab (but I didn't test it yet) :thinking: <|||||>> Hi @Narsil ,
> 
> the `warning.warn` to `raise RuntimeError` change in `src/transformers/convert_slow_tokenizer.py` breaks a lot of things: I wanted to fine-tune a mT5 model and it is now no longer possible (I'm using the PyTorch example from 

 How is it possible to rubustify it -> also DeBERTa v3 has byte fallback vocab (but I didn't test it yet) thinking


First of all we could revert by all means, but since now `tokenizers` has `ByteFallback` we could make it 1-1 for those, that was the idea behind upping to an error.

It's a relatively sizeable issue if there are models deployed out there which have inconsistent behavior regarding this though (slow using byte fallback, fast not using it). I'm not sure why it was a warning in the first place.


>  DeBERTa v3 

Let's have a look too.

As a user, what's your opinion here, should we just fix the various conversion scripts, or would you rather keep the warning with the previous pitfalls ?<|||||>Both are using Unigram with ByteFallback which isn't supported yet. <|||||>@Narsil After this commit `AutoTokenizer.from_pretrained` is extremely slow, spending time in `convert_slow_tokenizer.py` at every call. Is it expected? Or I am doing something wrong?<|||||>Which repo are you using? We need to create the fast files on the repo.


Converting from slow is super slow and there's nothing to be done about it (tokenizers needs to recreate a structure by doing O(n2) search over the vocab because spm does not store this information. <|||||>@ArthurZucker <|||||>I see thanks!