[docs] add a comment that offloading requires CUDA GPU (#35055)
* add commen to offloading
* Update docs/source/en/kv_cache.md
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
---------
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>