add reuters21578 dataset
CONTRIBUTOR
> Awesome !
> Good job on parsing these files :O
> 
> Do you think it would be hard to get the two other split configurations ?

It shouldn't be that hard, I think I can consider different config names for each split 
2020-07-28T13:33:18Z
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> > Awesome !
> > Good job on parsing these files :O
> > Do you think it would be hard to get the two other split configurations ?
> 
> It shouldn't be that hard, I think I can consider different config names for each split

Yes that would be perfect
2020-07-29T08:46:26Z
https://github.com/huggingface/datasets/pull/449#issuecomment-665527910
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CONTRIBUTOR
closing this PR and opening a new one to fix the circle CI problems
2020-08-03T11:10:25Z
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This PR adds the `Reuters_21578` dataset https://kdd.ics.uci.edu/databases/reuters21578/reuters21578.html  
#353 

The datasets is a lit of `.sgm` files which are a bit different from xml file indeed `xml.etree` couldn't be used to read files. I consider them as text file (to avoid using external library)  and read line by line (maybe there is a better way to do, happy to get your opinion on it)

In the Readme file 3 ways to split the dataset are given.:

- The Modified Lewis ("ModLewis") Split: train, test and unused-set

- The Modified Apte ("ModApte") Split : train, test and unused-set

- The Modified Hayes ("ModHayes") Split: train and test

Here I consider the last one as the readme file highlight that this split provides the ability to compare results with those of the 2 first splits.
