Add tokenizer config classes

[jackapbutler]

Fixes #990

I've just refactored out the tokenizer configuration details from the get_tokenizer function. This should hopefully;

• Make it easier to add new tokenizers without changing logic in get_tokenizer.
• Make it easier to see what get_tokenizer is doing without having to worry about the special tokens of each model type.

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[github-actions]

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Find more information in the repository's CONTRIBUTING.md

[sanagno]

Looks good although not too much more simple :))