Move `from_preset` to base tokenizer classes

[jbischof]

This PR would be an replication of #621 for our three tokenizer layers: WordPieceTokenizer, BytePairTokenizer, and SentencePieceTokenizer. Unlike #621 we do not need to create new classes but merely move some properties and methods to the existing ones.

Side note: Originally I wanted to move from_preset to tokenizer.Tokenizer, but preset loading differs for the three subclasses above.

Example steps for WordPieceTokenizer:

• Copy over following methods/properties to base class from BertTokenizer:
  • presets (return {})
  • from_preset with string formatting params to adapt to preset and class names
• Remove from_preset from each subclass and fill in the string params for from_preset docstring.

Following #621, add a mention of from_preset loading to the generic constructor docstring.

This process should be repeated for every XXTokenizer in the models/ folder.

[kanpuriyanawab]

Can I work on it?

[durganshyadav]

hey i would like can i work on this.