AutoTokenizer: infer the class from the tokenizer config if possible
# What does this PR do?

This PR adds the functionality to load a tokenizer with `AutoTokenizer.from_pretrained` after saving it locally (without saving the model config in the same folder).

To do this, the proper tokenizer class is saved in `tokenizer_config.json` and the `AutoTokenizer.from_pretrained` method will first look in this file before defaulting to the model config (like before).
