Function fp8_gemm_n128_mfast 

pub fn fp8_gemm_n128_mfast(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    input: DevicePtr,
    b_fp8: DevicePtr,
    output: DevicePtr,
    m: u32,
    n: u32,
    k: u32,
    stream: u64,
) -> Result<()>

2026-09-25: FP8-weight GEMM through the fp8_gemm_t_mfast kernel, with M on the fast grid axis (blockIdx.x = M block), so consecutive CTAs share a B panel.