Function w8a16_gemm_m16_n64 

pub fn w8a16_gemm_m16_n64(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    input: DevicePtr,
    weight: DevicePtr,
    block_scale: DevicePtr,
    output: DevicePtr,
    m: u32,
    n: u32,
    k: u32,
    stream: u64,
) -> Result<()>

N_TILE=64 twin of w8a16_gemm_m16 — identical arguments and identical per-output arithmetic, ceil(N/64) CTAs instead of ceil(N/32). Same signature, so a caller holds ONE ContiguousM16Gemm pointer and the tile is a dispatch choice, not a code path.