Function mla_kv_assemble_batched 

pub fn mla_kv_assemble_batched(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    kv_expanded: DevicePtr,
    k_rope_buf: DevicePtr,
    k_out: DevicePtr,
    v_out: DevicePtr,
    num_tokens: u32,
    nkv: u32,
    nope: u32,
    v_dim: u32,
    rope: u32,
    hd: u32,
    kv_expanded_stride: u32,
    stream: u64,
) -> Result<()>

2026-09-25: Per token, build K as [k_nope | k_rope] for each KV head (k_rope shared by all heads) and extract V from kv_expanded. blockIdx.y 0 builds K, 1 extracts V.