Function residual_add_rms_norm 

pub fn residual_add_rms_norm(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    hidden: DevicePtr,
    src: DevicePtr,
    weight: &DenseWeight,
    output: DevicePtr,
    residual: DevicePtr,
    num_tokens: u32,
    hidden_size: u32,
    eps: f32,
    stream: u64,
) -> Result<()>

2026-09-25: hidden += src, then output = rms_norm(hidden) and residual = hidden, in one kernel, one block per token row.