[fix] Ensure that the embeddings from hard negative mining are normalized

[tomaarsen]

Hello!

Pull Request overview

• Ensure that the embeddings from hard negative mining are normalized

Details

If you use use_faiss in mine_hard_negatives, then FAISS expects normalized embeddings. However, the models aren't necessarily normalized. This results in nan for larger datasets and really large scores for smaller datasets. This PR should resolve that by enforcing that all embeddings are normalized in mine_hard_negatives.

• Tom Aarsen