Function gemv_gate_up 

pub fn gemv_gate_up(
    gpu: &dyn GpuBackend,
    gate: &MlxInt8Weight,
    up: &MlxInt8Weight,
    x: DevicePtr,
    gate_y: DevicePtr,
    up_y: DevicePtr,
    stream: u64,
) -> Result<()>

2026-09-25: gate_y = gate x and up_y = up x in one launch of mlx_int8_gemv_gate_up, which reads x once for both. Both weights must share (out_features, in_features, group_size); only debug_assert_eq! checks it.