Function dense_gemm_prefill 

pub fn dense_gemm_prefill(
    gpu: &dyn GpuBackend,
    fallback_kernel: KernelHandle,
    pipelined_kernel: KernelHandle,
    input: DevicePtr,
    weight: &DenseWeight,
    output: DevicePtr,
    m: u32,
    n: u32,
    k: u32,
    stream: u64,
) -> Result<()>

Dense BF16 prefill GEMM. Prefer the pipelined tensor-core kernel when the selected target ships it, and retain the scalar kernel as an explicit compatibility fallback for older targets.