[XGLUE] Adding new dataset
Small poll  @thomwolf @yjernite @lhoestq @JetRunner @qiweizhen .

As stated in the XGLUE paper: https://arxiv.org/pdf/2004.01401.pdf , for each of the 11 down-stream tasks training data is only available in English, whereas development and test data is available in multiple different language *cf.* here: 

![Screenshot from 2020-11-04 15-02-17](https://user-images.githubusercontent.com/23423619/98120893-d7499a80-1eae-11eb-9d0b-57dfe5d4ee68.png)

So, I'd suggest to have exactly 11 "language-independent" configs: "ner", "pos", ... and give the sample in each dataset in the config a "language" label being one of "ar", "bg", .... => To me this makes more sense than making languaga specific config, *e.g.* "ner-de", ...especially because training data is only available in English. Do you guys agree? 
XGLUE is a multilingual GLUE like dataset propesed in this [paper](https://arxiv.org/pdf/2004.01401.pdf).

I'm planning on adding the dataset to the library myself in a couple of weeks.
Also tagging @JetRunner @qiweizhen in case I need some guidance 
[XGLUE] Adding new dataset 
 XGLUE is a multilingual GLUE like dataset propesed in this [paper](https://arxiv.org/pdf/2004.01401.pdf).

I'm planning on adding the dataset to the library myself in a couple of weeks.
Also tagging @JetRunner @qiweizhen in case I need some guidance  
 Small poll  @thomwolf @yjernite @lhoestq @JetRunner @qiweizhen .

As stated in the XGLUE paper: https://arxiv.org/pdf/2004.01401.pdf , for each of the 11 down-stream tasks training data is only available in English, whereas development and test data is available in multiple different language *cf.* here: 

![Screenshot from 2020-11-04 15-02-17](https://user-images.githubusercontent.com/23423619/98120893-d7499a80-1eae-11eb-9d0b-57dfe5d4ee68.png)

So, I'd suggest to have exactly 11 "language-independent" configs: "ner", "pos", ... and give the sample in each dataset in the config a "language" label being one of "ar", "bg", .... => To me this makes more sense than making languaga specific config, *e.g.* "ner-de", ...especially because training data is only available in English. Do you guys agree? 