Speedup modular conversion w/ multiproc

[muellerzr]

What does this PR do?

During my last PR, I found that make fix-copies was taking a ton of time. Trying to make it a bit faster by using multiproc instead. Let me know what we think.

Times calling time python utils/check_modular_conversion.py --fix_and_overwrite:

• Before (linear): 67s
• After (multiproc): 6s

(Mileage may vary with multiproc, of course)

Fixes # (issue)

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@ArthurZucker @Rocketknight1

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[qubvel]

I had the same thoughts just 15 minutes ago 😅 as far as I understand, files are topologically sorted, so the order of conversion matters, we should probably keep the dependency graph and apply multiprocessing level-by-level

[ArthurZucker]

Yep exactly!

[ArthurZucker]

Next level is to code it in rust :)