Return only embeddings from `generate_embeddings_batch/3`

In `Rag.Embedding.generate_embeddings_batch/3` we generate embeddings and put them into the ingestion map. This however is inconvenient when working with changesets instead of maps as you have to pull out the embedding from the map and put them into the changeset again.
So, we can instead just return the embeddings and let the user decide how to work with them.