Function kv_append 

pub fn kv_append(
    cache: &WgpuStorage,
    src: &WgpuStorage,
    h: usize,
    t: usize,
    hd: usize,
    cap: usize,
    len: usize,
)

Append src [h, t, hd] into cache [h, cap, hd] at row offset len.