add openwebtext2
It seems we need to `pip install jsonlines` to pass the checks ? Hi ! Do you really need `jsonlines` ? I think it simply uses `json.loads` under the hood.

Currently the test are failing because `jsonlines` is not part of the extra requirements `TESTS_REQUIRE` in setup.py

So either you can replace `jsonlines` with a simple for loop on the lines of the files and use `json.loads`, or you can add `TESTS_REQUIRE` to the test requirements (but in this case users will have to install it as well). Thanks for your suggestion. I now know `io` and json lines format better and has changed `jsonlines` to just `readlines`.
openwebtext2 is part of EleutherAI/The Pile, but AFAIK, The Pile dataset blend all sub datasets together thus we are not able to use just one of its sub dataset from The Pile data. So I create an independent dataset using The Pile preliminary components.

When I was creating dataset card. I found there is room for creating / editing dataset card. I've made it an issue. #2797

Also I am wondering whether the import of The Pile dataset is actively undertaken (because I may need it recently)? #1675