support loading model without config.json file

[itazap]

We already have support for loading a fast tokenizer with a tokenizer.model file only. However, we still require config.json to exist in the model folder (on hub or locally), even though it is not required to load from the model file. This feature will remove this dependency on config.json, allowing users to load from only a tokenizer.model file.

TESTS:

• updated stale tests
• updated existing test to work without config file
• added test in llama to load with only tokenizer.model file

Reviewer: @ArthurZucker

[HuggingFaceDocBuilderDev]

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