Function fused_k_norm_rope_cache_write_fp8 

pub fn fused_k_norm_rope_cache_write_fp8(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    k_in: DevicePtr,
    k_norm_weight: DevicePtr,
    positions: DevicePtr,
    k_cache_fp8: DevicePtr,
    slot_mapping: DevicePtr,
    num_tokens: u32,
    num_kv_heads: u32,
    head_dim: u32,
    rotary_dim: u32,
    block_size: u32,
    rms_eps: f32,
    theta: f32,
    inv_scale: f32,
    stream: u64,
) -> Result<()>

FP8-output sibling of fused_k_norm_rope_cache_write_bf16. Same semantics; one fewer BF16 round before the saturating FP8 cast.