Retaining unknown token behaver consistency in tokenizer for BERT and XLNET
I found the BERT tokenizer and XLNET tokenizer behave difference.
for example, `"His name is 燚"`
In BERT, it's tokenized as:` ['his', 'name', 'is', '[UNK]'],`
but in XLNET, it's tokenized as:` ['▁His', '▁name', '▁is', '▁', '燚']`
The difference may adverse to the uniform training frame for BERT and XLNET, and trouble the length-based processing.