Allow add_tokens for ESM (#28535)
* Allow non-special tokens to be added
* Add test, fix token adding code
* Revert changes to id_to_token and token_to_id
* Update the ESM tokenizer to be a bit more standardized
* Update src/transformers/models/esm/tokenization_esm.py
Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>
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Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>
