[Pegasus] Refactor Tokenizer (#8731)
* refactor
* further refactor
* fix the rest tomorrow
* save intermediate
* finish slow tokenizer
* make more tests pass
* finish refactor
* fix comment
* clean further
* fix name
* fix naming
* Update src/transformers/models/reformer/tokenization_reformer.py
* Apply suggestions from code review
* Apply suggestions from code review
* refactor
* fix init tokenizers
* refactor
* improve convert
* refactor
* correct convert slow tokenizer
* final fix for Pegasus Tok
* remove ipdb
* improve links
