Function nvfp4_scale_bf16 

pub fn nvfp4_scale_bf16(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    data: DevicePtr,
    scale: f32,
    total: u32,
    stream: u64,
) -> Result<()>

In-place ×scale2 for the down-projection MMQ output ([m, h] bf16).