Function nvfp4_mmq_quantize_act 

pub fn nvfp4_mmq_quantize_act(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    input_bf16: DevicePtr,
    out_y: DevicePtr,
    m: u32,
    k: u32,
    stream: u64,
) -> Result<()>

Quantize bf16 activations [m, k] -> block_fp4_mmq (e2m1 + ue4m3 group-16, ±2 scale search) into out_y. One thread per 16-value group; ne0 padded to 256.