Refactor script to reduce complexity

[milyiyo]

What does this PR do?

This PR refactors some functions of the script run_bart_dlm_flax.py.

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[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.

[sgugger]

Thanks for your PR. We prefer the current style for examples as in general users have indicated they prefer to:

• not have to look for intermediate functions but just read the code sequentially
• prefer if return xxx else return yyy statements to the suggested changes in this PR.

[milyiyo]

Thanks for the feedback, I will keep it in mind :)

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