compile and vmap can produce Sendable closures from Sendable inputs

[davidkoski]

Proposed changes

• this idea comes from Declare module weights as compile state in compiled decode paths mlx-swift-lm#589 (@aleroot)
• breaking change for compile calls that capture state without using inputs:

Checklist

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• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

[davidkoski]

Cleaner formatting. I was going to change grad in the same way, but it typically needs a model and I am not sure I can pass that through the most-inner closure that this uses. Keep as not-Sendable.