Is it possible to have a VecterStoreIndex retriever over a set of indices, rather than an entire query engine?
Basically I have 1 index per article, and I want to build a meta-index over them based on their article metadata and summaries, and be able to filter/retrieve top k article indices using vector search. ComposableGraph seems to only support as_query_engine() but not as_retriever(). My workaround would be to create a new Document (meta-doc) for each article index, containing their metadata only, and create an index+retriever over those and manually extract their index ids from the retrieved meta-docs...