Function w4a4_gemm 

pub fn w4a4_gemm(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    a_packed: DevicePtr,
    a_scale: DevicePtr,
    weight: &QuantizedWeight,
    output: DevicePtr,
    m: u32,
    n: u32,
    k: u32,
    stream: u64,
) -> Result<()>

2026-09-25: W4A4 GEMM on FP4 tensor cores (kernel w4a4_gemm). The activation comes from quantize_bf16_to_nvfp4 and is launched with scaleA2 = 1.0; the weight is an NVFP4 QuantizedWeight; the output is BF16 [M, N].