[FA4] Initial support

[vasqu]

Fixes #42405
#42404 has a lot of unnecessary things alongside it, this aims to supersede it

Remaining TODOs:

• Equivalence tests to fa3 (need to compile from source again 😢 ..in progress)
• Sanity check if the usage of partial destroys compile support for older fa versions
• Handle FA2 detection as it clashes with FA4 atm
• Refactor modeling utils? A lot of duplicated messages

[HuggingFaceDocBuilderDev]

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[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: edgetam, gemma3, modernbert, sam2, sam3, sam3_tracker