Add SAMSum Corpus dataset
also to fix the check_code_quality CI you have to remove the imports of the unused `csv` and `os` @lhoestq Thanks for the review! I have done what you asked, README is also updated. 🤗 
The CI fails because of the added dependency. I have never used circleCI before, so I am curious how will you solve that? I just added `py7zr` to our test dependencies merging since the CI is fixed on master Thanks! 🤗 
Did not spent much time writing README, might update later.

Copied description and some stuff from tensorflow_datasets
https://github.com/tensorflow/datasets/blob/master/tensorflow_datasets/summarization/samsum.py