Problems with WMT dataset
Hi! Yes, the docs are outdated. Expect this to be fixed soon. 

In the meantime, you can try to fix the issue yourself.

These are the configs/language pairs supported by `wmt15` from which you can choose:
* `cs-en` (Czech - English)
* `de-en` (German - English)
* `fi-en` (Finnish- English)
* `fr-en` (French - English)
* `ru-en` (Russian - English)

And the current implementation always uses all the subsets available for a language, so to define custom subsets, you'll have to clone the repo from the Hub and replace the line https://huggingface.co/datasets/wmt15/blob/main/wmt_utils.py#L688 with:
`for split, ss_names in (self._subsets if self.config.subsets is None else self.config.subsets).items()`

Then, you can load the dataset as follows:
```python
from datasets import load_dataset
dset = load_dataset("path/to/local/wmt15_folder", "<one of 5 available configs>", subsets=...) @mariosasko thanks a lot for the suggested fix! 
## Describe the bug
I am trying to load WMT15 dataset and to define which data-sources to use for train/validation/test splits, but unfortunately it seems that the official documentation at [https://huggingface.co/datasets/wmt15#:~:text=Versions%20exists%20for,wmt_translate%22%2C%20config%3Dconfig)](https://huggingface.co/datasets/wmt15#:~:text=Versions%20exists%20for,wmt_translate%22%2C%20config%3Dconfig)) doesn't work anymore.

## Steps to reproduce the bug
```shell
>>> import datasets
>>> a = datasets.translate.wmt.WmtConfig()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'datasets' has no attribute 'translate'
>>> a = datasets.wmt.WmtConfig()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'datasets' has no attribute 'wmt'
```

## Expected results
To load WMT15 with given data-sources.

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 2.0.0
- Platform: Linux-5.10.0-10-amd64-x86_64-with-glibc2.17
- Python version: 3.8.12
- PyArrow version: 7.0.0
- Pandas version: 1.4.1
