Fix BitsAndBytesConfig JSON serialization in TrainingArguments
Author: astefanutti
Pull request: yes

# What does this PR do?

This makes sure the BitsAndBytesConfig object gets converted to dict before TrainingArguments are serialized to JSON.

This happens when using BitsAndBytes and the TensorBoard integration for example. 

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Fixes #37518

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- [ ] Did you write any new necessary tests?

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