Add summarization template
> Nice thanks !
> Could you just move the test outside of the BaseDatasetTest class please ? Otherwise it will unnecessarily be run twice.

sure, on it! thanks for the explanations about the `self._to` method :) @lhoestq i've moved all the task template tests outside of `BaseDatasetTest` and collected them in their dedicated test case. (at some point i'll revisit this so we can just use `pytest` natively, but the PR is already getting out-of-scope :))
This PR adds a task template for text summarization. As far as I can tell, we do not need to distinguish between "extractive" or "abstractive" summarization - both can be handled with this template.

Usage:

```python
from datasets import load_dataset
from datasets.tasks import Summarization

ds = load_dataset("xsum", split="train")
# Dataset({
#     features: ['document', 'summary', 'id'],
#     num_rows: 204045
# })

summarization = Summarization(text_column="document", summary_column="summary")
ds.prepare_for_task(summarization)
# Dataset({
#     features: ['text', 'summary'],
#     num_rows: 204045
# })
```
