Fix non-kernel layers in liger `layers` module (API breakage)

[moon-bot-app]

Summary

The liger kernel's layers module currently contains non-kernel layers. These need to be separated out, which will constitute an API change requiring a version bump in build.toml.

Additionally, we should provide two interfaces: the original upstream-compatible one and a transformers-specific one, so that deviations from upstream don't cause trust issues in the kernel dev community.

Context

• Liger PR: liger-kernels: sync changes for transformers #906
• Related transformers PR: 🚨 [Kernels] Sync to latest version transformers#46039
• Discussion noted that layers is fair game for kernel layers but non-kernel layers need to be moved out

Slack context

https://huggingface.slack.com/archives/C085WUVD0LJ/p1748967485776759

cc @sayakpaul

[sayakpaul]

@danieldk this is probably not needed after #906?

[sayakpaul]

@vasqu this seems this will be solved with the syncs you're working on?

[vasqu]

Yep, I think we can close this. Syncing PR is also ready now and should be merged soon --> then applying the new ones