add pg19 dataset
@lucidrains - Thanks a lot for making the PR - PG19 is a super important dataset! Thanks for making it. Many people are asking for PG-19, so it would be great to have that in the library as soon as possible @thomwolf . @mariamabarham yup! around 11GB! I'm looking forward to our first deep learning written novel already lol. It's definitely happening Good to merge IMO. Oh I just noticed but as we changed the urls to download the files, we have to update `dataset_infos.json`.
Could you re-rurn `nlp-cli test ./datasets/pg19 --save_infos` ? @lhoestq on it! should be good! @lhoestq - I think it's good to merge no? `dataset_infos.json` is still not up to date with the new urls (we can see that there are urls like `gs://deepmind-gutenberg/train/*` instead of `https://storage.googleapis.com/deepmind-gutenberg/train/*` in the json file)

Can you check that you re-ran the command to update the json file, and that you pushed the changes @lucidrains ? @lhoestq ohhh, I made the change in this commit https://github.com/lucidrains/nlp/commit/f3e23d823ad9942031be80b7c4e4212c592cd90c , that's interesting that the pull request didn't pick it up. maybe it's because I did it on another machine, let me check and get back to you! @lhoestq wrong branch 😅 thanks for catching!  Awesome thanks 🎉
https://github.com/huggingface/nlp/issues/274

Add functioning PG19 dataset with dummy data

`cos_e.py` was just auto-linted by `make style`