Custom embedding model for the retriever. (Bring your own embedding)
It needs to match the embedding model that was used to embed docs in the datastore.
It needs to be a langchain embedding VertexAIEmbeddings(project="{PROJECT}")
If you provide an embedding model, you also need to provide a ranking_expression
and a custom_embedding_field_path.
https://cloud.google.com/generative-ai-app-builder/docs/bring-embeddings
