How to use the Hydra 2.0 API to parse geomSubset types

Community Hydra
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At present, I am using the Hydra 1.0 API for parsing .usd files. Recently, I learned that upgrading to Hydra 2.0 API has brought many powerful features (thank you to all developers for your contributions). I want to migrate the parsing logic to the Hydra 2.0 API, but I have encountered difficulties.
Specifically, I created a sphere using houdini and divided its material into upper and lower parts using geomeSubset. This .usd file is used to test the parsing of geomeSubset, and the relevant code is as follows:

"name": "HD_ENABLE_SCENE_INDEX_EMULATION",
"value": "true"

void MyRenderDelegate::SetTerminalSceneIndex(const HdSceneIndexBaseRefPtr &terminalSceneIndex) {
    HdRenderDelegate::SetTerminalSceneIndex(terminalSceneIndex);
    sceneIndex = terminalSceneIndex;
    if (!observer) { observer = std::make_unique<MySceneObserver>(this); }
    if (terminalSceneIndex) {
        terminalSceneIndex->AddObserver(HdSceneIndexObserverPtr{observer.get()});
    }
}

void MySceneObserver::TraverseRecursively(
    const HdSceneIndexBaseRefPtr& sceneIndex,
    const SdfPath& currentPath
) {
    const auto scene_prim = sceneIndex->GetPrim(currentPath);
    if (scene_prim.dataSource) {
        const auto flattening_scene_index = HdFlatteningSceneIndex::New(sceneIndex, scene_prim.dataSource);
        const auto flattening_scene_prim = flattening_scene_index->GetPrim(currentPath);
        const auto flattening_child_prim_paths = flattening_scene_index->GetChildPrimPaths(currentPath);
        if (flattening_scene_prim.primType == HdPrimTypeTokens->geomSubset) {
            parse_geom_subset(currentPath, flattening_scene_prim);
        } else if (flattening_scene_prim.primType == HdPrimTypeTokens->mesh) {
            parse_mesh(currentPath, flattening_scene_prim);
        } else if (flattening_scene_prim.primType == HdPrimTypeTokens->instancer) {
            parse_instancer(currentPath, flattening_scene_prim);
        } else if (flattening_scene_prim.primType == HdPrimTypeTokens->camera) {
            parse_camera(currentPath, flattening_scene_prim);
        } else if (
            flattening_scene_prim.primType == HdPrimTypeTokens->distantLight
            || flattening_scene_prim.primType == HdPrimTypeTokens->sphereLight
            || flattening_scene_prim.primType == HdPrimTypeTokens->rectLight
            || flattening_scene_prim.primType == HdPrimTypeTokens->diskLight
            || flattening_scene_prim.primType == HdPrimTypeTokens->cylinderLight
            || flattening_scene_prim.primType == HdPrimTypeTokens->domeLight
        ) {
            parse_light(currentPath, flattening_scene_prim);
        }
    }

    const auto childPaths = sceneIndex->GetChildPrimPaths(currentPath);
    for (const auto& childPath : childPaths) {
        TraverseRecursively(sceneIndex, childPath);
    }
}

I have previewed the file using usdview and I am sure there is a mesh type inside, and there are two geomSubset types under that mesh. However, I can only traverse to the HdPrimTypeTokens->mesh type and retrieve its data, this code cannot traverse the HdPrimTypeTokens->geomSubset type.

I have looked up some information about Hydra 2.0, and maybe I should inherit HdSceneIdexObserver and rewrite the PrimsAdd function. However, I have tried but have not been able to get the data in the HdPrimTypeTokens->geomSubset type.

PS:
OS: Arch Linux x86_64
OpenUSD: 25.08
The .usd file: geo_subset.usda (18.0 KB)

Reference