model : avoid ggml_cont_3d for fused QKV weights (#15662)
* model : avoid ggml_cont_3d for fused QKV weights
ggml-ci
* kv-cache : make cpy_k and cpy_v implementation more readable
ggml-ci
* cont : add comments
ggml-ci
* cont : minor fix [no ci]
* cont : one more fix
* cont : clarity
ggml-ci
* kv-cache : require contiguous heads of k_cur and v_cur
ggml-ci