Add CpmTokenizerFast
# What does this PR do?
Add a fast version of `CpmTokenizer`

Fixes #12837
> I don't think the fast tokenizer as it's written works for now, as the fast tokenizer do not call the `_tokenize` method.

Oops! It looks the old pull request isn't right. I'll take a closer look<|||||>@sgugger I've updated and tested it. It works fine - only needs to wait for the `tokenizer.json` to be uploaded.<|||||>Tokenizer file uploaded. Merging it.