rotary: Refactor split into separate files + warning in readme

[vasqu]

As per title, successor to #929 but using direct branches and not a fork to have proper CI

• Adds a warning to the card as the kernel is outdated / not maintained anymore
• Split the func specifics to layer (should be done sooner or later)

[danieldk]

The build currently fails:

       >   File "/nix/store/009nfcf16mly93zr54yywrjisv1ngpbj-rotary-torch-ext/torch212-cxx11-cu126-x86_64-linux/__init__.py", line 4, in <module>
       >     from .layers import apply_rotary_transformers
       >   File "/nix/store/009nfcf16mly93zr54yywrjisv1ngpbj-rotary-torch-ext/torch212-cxx11-cu126-x86_64-linux/layers.py", line 3, in <module>
       >     from . import apply_rotary
       > ImportError: cannot import name 'apply_rotary' from '_rotary_cuda_73f4e9f' (/nix/store/009nfcf16mly93zr54yywrjisv1ngpbj-rotary-torch-ext/torch212-cxx11-cu126-x86_64-linux/__init__.py)

[vasqu]

Will check, probably some wrong relative import. Should be straightforward