Adding PolEval2019 Machine Translation Task dataset
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**NOTE:**

- Train and Dev: Manually downloaded (auto download is repeatedly giving `ConnectionError` for one of the files), Test: Auto Download
- Dummy test is passing
- The json file has been created with hard-coded paths for the manual downloads _(hardcoding has been removed from the final uploaded script)_
- datasets-cli is still **failing** . It is not picking the right directory for the config. For instance, my folder structure is as below:
 ```
            ~/Downloads/Data/
             |--- English-to-Polish
                   |--- (corresponding files) 
             |--- Russian-Polish
                   |--- (corresponding files) 
```

When ru-pl is selected, ideally it has to search in Russian-Polish folder, but it is searching in '/Downloads/Data/' folder and hence getting a FileNotFound error.

The command run is 
`python datasets-cli test datasets/poleval2019_mt/ --save_infos --all_configs --data_dir ~/Downloads/Data/
`
 Hi !
Thanks for the changes :)

The only error left is the dummy data. Since we changed for standard downloads instead of manual downloads its structure changed. Fortunately you can auto-generate the dummy data with this command:

```
datasets-cli dummy_data ./datasets/poleval2019_mt --auto_generate --match_text_files "*"
```

Can you regenerate the dummy data using this command please ? Thank you for the help @lhoestq !! I was generating the dummy dataset in a wrong way! That _--match_text_files "*"_ did the trick! Now all the tests have passed! :-)
Facing an error with pytest in training. Dummy data is passing.
README has to be updated.
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