Add Hippocorpus Dataset
> Before we merge can you try to reduce the size of the dummy_data.zip file ?
> 
> To do so feel free to only keep a few lines of the csv files ans also remove unnecessary chunks of texts (for example keep only the first sentences of a story).

Hi @lhoestq, I have reduced the size of the dummy_data.zip file by making the necessary changes you had suggested.  merging since the CI is fixed on master