Fix tokenizers pretrained saving/loading
Fix #5571

I managed to fix the saving part, but I don't really know what to do for the loading part (in `_from_pretrained`).
Hey @n1t0, great job, saving works now! :)
Do you think `AddedToken` instances should be serialized with additional meta data? 

I managed to tweak JSON (de)serialization with custom hooks to process objects properly but it is difficult to infer the deserializing type from a single set of attributes, passing `__class__` meta data helps to make guesses at what types to deserialize. <|||||>Superseded by #6026