Function qsa_qprep_rows 

pub fn qsa_qprep_rows(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    qk: DevicePtr,
    q_norm_w: DevicePtr,
    q_out: DevicePtr,
    rows: u32,
    first_pos: u32,
    qkw: u32,
    n_heads: u32,
    hd: u32,
    rot: u32,
    theta: f32,
    eps: f32,
    stream: u64,
) -> Result<()>

Stage 2: per-row q prep for a contiguous selective row range.