Issue: SyntaxError with WMT datasets

The following snippet produces a syntax error:

```
import nlp

dataset = nlp.load_dataset('wmt14')
print(dataset['train'][0])
```

```
Traceback (most recent call last):

  File "/home/tom/.local/lib/python3.6/site-packages/IPython/core/interactiveshell.py", line 3326, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)

  File "<ipython-input-8-3206959998b9>", line 3, in <module>
    dataset = nlp.load_dataset('wmt14')

  File "/home/tom/.local/lib/python3.6/site-packages/nlp/load.py", line 505, in load_dataset
    builder_cls = import_main_class(module_path, dataset=True)

  File "/home/tom/.local/lib/python3.6/site-packages/nlp/load.py", line 56, in import_main_class
    module = importlib.import_module(module_path)

  File "/usr/lib/python3.6/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)

  File "<frozen importlib._bootstrap>", line 994, in _gcd_import

  File "<frozen importlib._bootstrap>", line 971, in _find_and_load

  File "<frozen importlib._bootstrap>", line 955, in _find_and_load_unlocked

  File "<frozen importlib._bootstrap>", line 665, in _load_unlocked

  File "<frozen importlib._bootstrap_external>", line 678, in exec_module

  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed

  File "/home/tom/.local/lib/python3.6/site-packages/nlp/datasets/wmt14/c258d646f4f5870b0245f783b7aa0af85c7117e06aacf1e0340bd81935094de2/wmt14.py", line 21, in <module>
    from .wmt_utils import Wmt, WmtConfig

  File "/home/tom/.local/lib/python3.6/site-packages/nlp/datasets/wmt14/c258d646f4f5870b0245f783b7aa0af85c7117e06aacf1e0340bd81935094de2/wmt_utils.py", line 659
    <<<<<<< HEAD
     ^
SyntaxError: invalid syntax
```

Python version:
`3.6.9 (default, Apr 18 2020, 01:56:04)  [GCC 8.4.0]`
Running on Ubuntu 18.04, via a Jupyter notebook

Comments:
Jeez - don't know what happened there :D Should be fixed now! 

Thanks a lot for reporting this @tomhosking !

Hi @patrickvonplaten!

I'm now getting the below error:

```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-28-3206959998b9> in <module>
      1 import nlp
      2 
----> 3 dataset = nlp.load_dataset('wmt14')
      4 print(dataset['train'][0])

~/.local/lib/python3.6/site-packages/nlp/load.py in load_dataset(path, name, version, data_dir, data_files, split, cache_dir, download_config, download_mode, ignore_verifications, save_infos, **config_kwargs)
    507     # Instantiate the dataset builder
    508     builder_instance = builder_cls(
--> 509         cache_dir=cache_dir, name=name, version=version, data_dir=data_dir, data_files=data_files, **config_kwargs,
    510     )
    511 

TypeError: Can't instantiate abstract class Wmt with abstract methods _subsets
```



To correct this error I think you need the master branch of `nlp`. Can you try to install `nlp` with. `WMT` was not included at the beta release of the library. 

Can you try:
`pip install git+https://github.com/huggingface/nlp.git`

and check again? 

That works, thanks :)

The WMT datasets are listed in by `list_datasets()` in the beta release on pypi - it would be good to only show datasets that are actually supported by that version?

Usually, the idea is that a dataset can be added without releasing a new version. The problem in the case of `WMT` was that some "core" code of the library had to be changed as well. 

@thomwolf @lhoestq @julien-c - How should we go about this. If we add a dataset that also requires "core" code changes, how do we handle the versioning? The moment a dataset is on AWS it will actually be listed with `list_datasets()` in all earlier versions...

Is there a way to somehow insert the `pip version` to the HfApi() and get only the datasets that were available for this version (at the date of the release of the version) @julien-c ? 

We plan to have something like a `requirements.txt` per dataset to prevent user from loading dataset with old version of `nlp` or any other libraries. Right now the solution is just to keep `nlp` up to date when you want to load a dataset that leverages the latests features of `nlp`.

For datasets that are on AWS but that use features that are not released yet we should be able to filter those from the `list_dataset` as soon as we have the `requirements.txt` feature on (filter datasets that need a future version of `nlp`).

Shall we rename this issue to be more explicit about the problem ?
Something like `Specify the minimum version of the nlp library required for each dataset` ?

Closing this one.
Feel free to re-open if you have other questions :)