Function mamba2_ssd_cumsum 

pub fn mamba2_ssd_cumsum(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    dt_raw: DevicePtr,
    a_log: DevicePtr,
    dt_bias: DevicePtr,
    dt_out: DevicePtr,
    da_cs: DevicePtr,
    seq_len: u32,
    num_heads: u32,
    nchunks: u32,
    batch_size: u32,
    dt_stride: u32,
    dt_min: f32,
    dt_max: f32,
    stream: u64,
) -> Result<()>

K1: per-chunk dt (softplus+clamp) and inclusive cumsum of the log-decay.