add amazon polarity dataset
`amazon_polarity` is probably a subset of `amazon_us_reviews` but I am not entirely sure about that.
I guess `amazon_polarity` will help in reproducing results of papers using this dataset since even if it is a subset from `amazon_us_reviews`, it is not trivial how to extract `amazon_polarity` from `amazon_us_reviews`, especially since `amazon_us_reviews` was released after `amazon_polarity`.  do you know what the problem would be ? should I pull the master before ? @lhoestq  The error just appeared on master. I will try to fix it today.
You can ignore them since it's not related to the dataset you added merging since the CI is fixed on master Great thanks for the help. 
This corresponds to the amazon (binary dataset) requested in https://github.com/huggingface/datasets/issues/353