refactor: formalize artifact saving

[gsprochette]

Description

Formalize artifact saving and expose it to more algorithms, and add artifact hooks into Smash config

• Generalized artifact saving/loading: Extracted and formalized the artifact-saving logic so it is no longer tied specifically to the Torch Compile workflow. Other Pruna algorithms can now register and reuse the same standardized artifact save/load utilities.
• Config-driven hooks: Extended the Smash configuration to include save_artifacts_fns and load_artifacts_fns, working in a way similar to save_fns and load_fns
• Improved composability: This structure makes it easy to save artifacts (such as parameters estimated from a warmup, or checkpoints for finetuning or distillation).

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Additional Notes

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[llcnt]

Nice work, this is a very cool feature :) (I was personally doing a custom save/load artifacts in my ongoing PR )

[simlang]

looks good! Thanks for taking care of that, super useful for literally everything 🚀

Just had a question about the backward compatibility

[simlang]

i see why this is necessary, but it would be nice to remove this as soon as possible again - should we add a deprecation warning or something, and then actually remove it again at some point?