FAISS in OpenSearch: Support HNSW for cosine #2913
Description
As a user I want to use FAISS's HNSW for cosine similarity too with OpenSearchDocumentStore.
According to https://opensearch.org/docs/latest/search-plugins/knn/knn-index/ FAISS plug-in only supports dot product and l2. So we have to normalize vectors before indexing/searching.
- normalize vectors manually at index time
- normalize vectors manually at query time
- use dot_product space under the hood