Data mixer Integration

[August-murr]

What does this PR do?

Fixes #2112
just putting this up as a demo. I didn’t get feedback on whether this should be part of trl or datasets, so I tried integrating a data mixer using a JSON config file.

• There are a few caveats—like the JSON file might not be beginner-friendly, and there aren’t any default values, so things can break if the config isn’t right.
• It’s currently limited to CLI usage since It was prioritized. Let me know if you’d like it extended for other use cases.
• I’m not sure if I put the functions/files in the right locations, so feel free to suggest better locations.

I’ve also included this script to show that it works. Let me know what you think or if this should be structured differently!

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

@lewtun