Preserve `list` type of `additional_special_tokens` in `special_token_map` (#12759)
* preserve type of `additional_special_tokens` in `special_token_map`
* format
* Update src/transformers/tokenization_utils_base.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>