llama : rotate activations for better quantization (#21038)

llama : rotate activations for better quantization (#21038)

* llama : rotate activations for better quantization

* cont : rotate V more + refactor

* cont : rotate caches separately + support non-power-of-2 head sizes

* cont : simplify

* cont : add reference for V rotation

* cont : refactor

* cont : support context shift

* cont : consolidate

* cont : dedup + allow different types for the rotation matrix

* cont : add env variable to disable rotation

* cont : simplify attn rot kv cache logic + rename env

* cont : pre-compute the Hadamard matrices