Add TAPAS trained on NQ

The authors of the "Open Domain Question Answering over Tables via Dense Retrieval" papers released the weights for TAPAS retriever trained on NQ here: https://github.com/google-research/tapas/blob/master/DENSE_TABLE_RETRIEVER.md

It would be nice if those weights could be converted to Huggingface model (I would presume it's fairly similar the other finetuned model since they share the same architecture, and I'd be happy to do it myself if there's some scripts I can run)

Start with the TAPAS DENSE_TABLE_RETRIEVER.md document linked in the issue and inspect how the released NQ retriever weights differ from the existing Hugging Face TAPAS model. Convert and expose those weights as a Hugging Face model, with successful loading and use of the NQ-trained retriever as the completion criterion.

Written by the indexing model from the issue text.