convert : support loading vocab from fast tokenizer config (#3633)

* Add HFVocab into convert.py

* Update convert.py

* Update convert.py

* add bytes_to_unicode function

* change add_meta_vocab fucntion

* remove debug code

* remove byte_encoder

* Add newline between classes

* Check tokenizer.json when tokenizer.model is not exist.

* Move transformers dependency to local code

* Add error context with 'raise from'

* Add fast tokenizer option to BpeVocab

* Update convert.py

* Add VocabLoader and remove *Vocab class

* Add transformers dependency

* remove added tokens and check newline token to decide spm or bpe

* Update convert.py

* Add special token type

* Update convert.py

* Update convert.py

* Update convert.py

* Fix typo in convert.py

* Fix when params.n_vocab < tokenizer vocab size

* update vocab class

* change funtion name

* Remove unused variable/functions, add types to class variable and methods, delete blank liens

* fix flake8 warnings

* code style cleanup

* make mypy happy

* change exception

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Co-authored-by: Jared Van Bortel <[email protected]>