wmt19 is broken

1. Check which lang pairs we have: `--dataset_name wmt19`:

Please pick one among the available configs: ['cs-en', 'de-en', 'fi-en', 'gu-en', 'kk-en', 'lt-en', 'ru-en', 'zh-en', 'fr-de']

 
2. OK, let's pick `ru-en`:

`--dataset_name wmt19 --dataset_config "ru-en"`

no cookies:

```
Traceback (most recent call last):
  File "./run_seq2seq.py", line 661, in <module>
    main()
  File "./run_seq2seq.py", line 317, in main
    datasets = load_dataset(data_args.dataset_name, data_args.dataset_config_name)
  File "/mnt/nvme1/code/huggingface/datasets-master/src/datasets/load.py", line 740, in load_dataset
    builder_instance.download_and_prepare(
  File "/mnt/nvme1/code/huggingface/datasets-master/src/datasets/builder.py", line 572, in download_and_prepare
    self._download_and_prepare(
  File "/mnt/nvme1/code/huggingface/datasets-master/src/datasets/builder.py", line 628, in _download_and_prepare
    split_generators = self._split_generators(dl_manager, **split_generators_kwargs)
  File "/home/stas/.cache/huggingface/modules/datasets_modules/datasets/wmt19/436092de5f3faaf0fc28bc84875475b384e90a5470fa6afaee11039ceddc5052/wmt_utils.py", line 755, in _split_generators
    downloaded_files = dl_manager.download_and_extract(urls_to_download)
  File "/mnt/nvme1/code/huggingface/datasets-master/src/datasets/utils/download_manager.py", line 276, in download_and_extract
    return self.extract(self.download(url_or_urls))
  File "/mnt/nvme1/code/huggingface/datasets-master/src/datasets/utils/download_manager.py", line 191, in download
    downloaded_path_or_paths = map_nested(
  File "/mnt/nvme1/code/huggingface/datasets-master/src/datasets/utils/py_utils.py", line 233, in map_nested
    mapped = [
  File "/mnt/nvme1/code/huggingface/datasets-master/src/datasets/utils/py_utils.py", line 234, in <listcomp>
    _single_map_nested((function, obj, types, None, True)) for obj in tqdm(iterable, disable=disable_tqdm)
  File "/mnt/nvme1/code/huggingface/datasets-master/src/datasets/utils/py_utils.py", line 190, in _single_map_nested
    mapped = [_single_map_nested((function, v, types, None, True)) for v in pbar]
  File "/mnt/nvme1/code/huggingface/datasets-master/src/datasets/utils/py_utils.py", line 190, in <listcomp>
    mapped = [_single_map_nested((function, v, types, None, True)) for v in pbar]
  File "/mnt/nvme1/code/huggingface/datasets-master/src/datasets/utils/py_utils.py", line 172, in _single_map_nested
    return function(data_struct)
  File "/mnt/nvme1/code/huggingface/datasets-master/src/datasets/utils/download_manager.py", line 211, in _download
    return cached_path(url_or_filename, download_config=download_config)
  File "/mnt/nvme1/code/huggingface/datasets-master/src/datasets/utils/file_utils.py", line 274, in cached_path
    output_path = get_from_cache(
  File "/mnt/nvme1/code/huggingface/datasets-master/src/datasets/utils/file_utils.py", line 584, in get_from_cache
    raise FileNotFoundError("Couldn't find file at {}".format(url))
FileNotFoundError: Couldn't find file at https://storage.googleapis.com/tfdataset-data/downloadataset/uncorpus/UNv1.0.en-ru.tar.gz
```

Labels: name: dataset bug

This was also mentioned in https://github.com/huggingface/datasets/issues/488 

The bucket where is data was stored seems to be unavailable now. Maybe we can change the URL to the ones in https://conferences.unite.un.org/uncorpus/en/downloadoverview ?

Closing since this has been fixed by #1912