Function residual_add 

pub fn residual_add(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    residual: DevicePtr,
    src: DevicePtr,
    num_elements: u32,
    stream: u64,
) -> Result<()>

BF16 residual add: residual[i] += src[i] (in-place).

Kernel: bf16_residual_add(residual, src, n) Grid: (ceil(n/256), 1, 1) Block: (256, 1, 1)