Function w4a4_gemm_mfast 

pub fn w4a4_gemm_mfast(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    a_packed: DevicePtr,
    a_scale: DevicePtr,
    weight: &QuantizedWeight,
    output: DevicePtr,
    m: u32,
    n: u32,
    k: u32,
    stream: u64,
) -> Result<()>

W4A16 GEMM with N_TILE=128: same kernel signature, wider N tile.

Grid: (ceil(N/128), ceil(M/64), 1) Block: (128, 1, 1) w4a4_gemm_mfast: same W4A4 GEMM with M on the fast grid axis, so the M-blocks sharing a B panel are co-resident and B streams from DRAM once.