The DPR embeddings are trained so that the pairs of questions/contexts maximize the inner product.
When they first released the official DPR code, the maximum inner product metric was not available in faiss for HNSW yet. Therefore they used a trick to make the L2 index work to maximise inner product by using this trick 2 that adds an extra dimension.
However times have changed and now HNSW supports the maximum inner product metric so you don’t need to use this trick anymore. You can just specify faiss.METRIC_INNER_PRODUCT inside the HNSW arguments.