Add wiki dpr multiset embeddings
I still have to compute the dataset_infos, and build + host the indexes update: I'm computing the metadata, will update the PR soon Finally all green and ready to merge :)
There are two DPR encoders, one trained on Natural Questions and one trained on a multiset/hybrid dataset.
Previously only the embeddings from the encoder trained on NQ were available. I'm adding the ones from the encoder trained on the multiset/hybrid dataset.
In the configuration you can now specify `embeddings_name="nq"` or `embeddings_name="multiset"`