[Datasets] add ted_hrlr
@thomwolf - After looking at `xnli` I think it's better to leave the translation features and add a `translation` key to make them work in our framework. 

The result looks like this:
![Screenshot from 2020-05-12 18-34-43](https://user-images.githubusercontent.com/23423619/81721933-ee1faf00-9480-11ea-9e95-d6557cbd0ce0.png)

you can see that each split has a `translation` key which value is the nlp.features.Translation object. 

That's a simple change. If it's ok for you, I will add dummy data for the other configs and treat the other translation scripts in the same way.