`wiki_dpr` details for Open Domain Question Answering tasks
Hi ! According to the DPR paper, the wikipedia dump is the one from Dec. 20, 2018.
Each instance contains a paragraph of at most 100 word, as well as the title of the wikipedia page it comes from and the DPR  embedding (a 768-d vector). Closed by:
- #3534
Hey guys!

Thanks for creating the `wiki_dpr` dataset!

I am currently trying to use the dataset for context retrieval using DPR on NQ questions and need details about what each of the files and data instances mean, which version of the Wikipedia dump it uses, etc. Please respond at your earliest convenience regarding the same! Thanks a ton!

P.S.: (If one of @thomwolf @lewtun @lhoestq could respond, that would be even better since they have the first-hand details of the dataset. If anyone else has those, please reach out! Thanks!)