add MS MARCO dataset
The dummy data for v2.1 is missing as far as I can see. I think running the  dummy data command should work correctly here.  Also, it might be that the structure of the dummy data is wrong - looking at `generate_examples` the structure does not look too easy. The fact that the dummy data for v2.1 is missing shouldn't make the test fails I think. But as you mention the dummy data structure of v1.1 is wrong. I tried to rename files but it does not solve the issue. Is MS mARCO added to nlp library?I am not able to view it? > Is MS mARCO added to nlp library?I am not able to view it?

Hi @parthplc ,the PR is  not merged yet. The dummy data structure is still failing. Maybe @patrickvonplaten can help with it. Dataset is fixed and should be ready for use. @mariamabarham @lhoestq feel free to merge whenever! > Dataset is fixed and should be ready for use. @mariamabarham @lhoestq feel free to merge whenever!

thanks
This PR adds the MS MARCO dataset as requested in this issue #336. MS mARCO has multiple task including:

- Passage and Document Retrieval

- Keyphrase Extraction

- QA and NLG

This PR only adds the 2 versions of the  QA and NLG task dataset which was realeased with the original paper here https://arxiv.org/pdf/1611.09268.pdf 

Tests are failing because of the dummy data. I tried to fix it without success. Can you please have a look at it? @patrickvonplaten , @lhoestq 