Possibility to use WhitespaceSplit as pre_tokenizer instead of BPE/Sentencepiece?
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## Details
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I was customizing a tokenizer with vocab and merges from tokenizers module. However when running intended Language Modelling task with the customized tokenizer, the class in transformers repository(PretrainedTokenizer) is not the same. I was trying to run a roberta-for-masked-lm objective with whole word tokenization scheme but seems if I were to  RobertaTokenizer, it cannot support changing the 'pre_tokenizer' scheme. May I know in such case what shall I do

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