Function residual_add 

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

2026-09-25: In-place BF16 residual[i] += src[i] (kernel bf16_residual_add).