Update tokenization & transformers to 3.4.0

[mozharovsky]

Summary

This PR updates 🤗/transformers to version 3.4.0 and restructures the tokenization block around fast tokenizers.

Patch Notes

• Generated stubs for 🤗/transformers to avoid dummy classes in code analysis (Pylance)
• Moved BBPE tokenizer implementation from TransformerTokenizerModule -> ByteLevelBPETokenizerModule
• Moved base tokenizer module implementation to TransformerTokenizerModule
• Renamed TransformerTokenizerFast -> ByteLevelBPETokenizerFast
• Renamed tokenizer module registered component transformer-tokenizer-fast -> byte-level-bpe-tokenizer
• Renamed CodeTokenizerFast -> CodeBBPETokenizerFast
• Renamed CodeTokenizerModule -> CodeBBPETokenizerModule
• Renamed tokenizer module registered component code-tokenizer-fast -> code-bbpe-tokenizer

[mozharovsky]

Next we're gonna rework the pre-trained tokenizers (de)serialization across the library. More details to come.