Add nq_open question answering dataset
@SBrandeis  thanks for the review, I applied your suggested changes, but CI is failing now not sure about the error. Many thanks @Nilanshrajput  !
The failing tests on CI are not related to your changes, merging master on your branch should fix them :)
If you're interested in what causes the CI to fail, checkout [this commit](https://github.com/huggingface/datasets/commit/9a0f1e20ca1e783cb14c1ab1cc2f54b0b5b201e8) @SBrandeis done!
 Hello @Nilanshrajput, your PR includes changes from other branches too now (485 files changed)
Would you mind creating another branch from master with your changes and opening a new PR? @SBrandeis sorry i messed up the git history, #1587 I opened this new pr!  closing in favor of #1587 
Added nq_open Open-domain question answering dataset.

The NQ-Open task is currently being used to evaluate submissions to the EfficientQA competition, which is part of the NeurIPS 2020 competition track.