Implement custom Chinese tokenizer.

[emfomy]

We may implement our own tokenizer rather than using BertTokenizerFast.
Our own tokenizer should have the following features:

• Disable word piece. Convert text to token ID character by character (e.g. tokenizer.convert_tokens_to_ids(list(input_text)))
• Reimplement clean_up_tokenization method. The default method is implemented for English only. Our method may remove whitespaces and convert half-width punctuations to full-width ones.

[emfomy]

Consider using the 🤗Tokenizers after it release a stable version instead.