Return offset mapping using tokenizer

Hi guys, awesome work with this project, it is helping us a lot! Is it possible to add the parameter "return_offset_mapping" to the Tokenizer class, just like in the Transformers python library?

Reason for request

The returned offsets are crucial for our project in Named Entity Recognition, as we need to associate the predicted tags with the original sentence (prior to tokenization). Currently we can only associate the predicted tags with the list of subtokens, as they are a 1-to-1 association, and we know that the list of subtokens ignore some elements, such as whitespaces, which are present in the original sentence.

Additional context

Reference for "return_offset_mapping" on Transformers python library: https://huggingface.co/docs/transformers/v4.35.2/en/main_classes/tokenizer#transformers.PreTrainedTokenizerFast.__call__.return_offsets_mapping