FastTokenizer for LLaMa
### Feature request

FastTokenizer support for LLaMa sentencepiece tokenizer.

### Motivation

The offset_mapping is only available in FastTokenizer, it would be useful if there's support for this.

### Your contribution

I have tried using existing sentencepiece based model as replacement. However hf conversation code means we are missing the byte fallback support 

```
The sentencepiece tokenizer that you are converting to a fast tokenizer uses the byte fallback option which is not implemented in the fast tokenizers
```

Which means out of vocabulary tokens are simply mapped to <unk> instead of using the byte mapping inside the vocab.
Let's maybe wait for the LLaMa PR to be merged first?<|||||>it is fix on tokenizers

https://github.com/huggingface/tokenizers/pull/1183<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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