Function run_multigpu_hls 

pub async fn run_multigpu_hls(
    params: MultiGpuParams<'_>,
    sink: Arc<dyn ProgressSink>,
) -> Result<Vec<Option<RungManifest>>>

Run the reactive multi-GPU variant phase. Returns one Option<RungManifest> per rung (in rung order); None means the rung produced no segments.