the data provided by Li Dong et al. were already tokenized, which means
that they are not compatible with all the models in the library. We
thus process the raw data directly and tokenize them using the models'
tokenizers.
We write a function to load an preprocess the CNN/Daily Mail dataset as
provided by Li Dong et al. The issue is that this dataset has already
been tokenized by the authors, so we actually need to find the original,
plain-text dataset if we want to apply it to all models.
