Update metadata if dataset features are modified
@lhoestq I'll try to add a test later if you think this approach with the wrapper is good. Awesome thank you !
Yes this approach with a wrapper is good :) @lhoestq Added a test. To verify that this change fixes the problem, replace:
```
!pip install datasets==1.5
```
with:
```
!pip install git+https://github.com/mariosasko/datasets-1.git@update-metadata
```
in the first cell of the notebook that is attached to the linked issue.

The CI failure is unrelated I think (building the docs locally doesn't throw an error). The CI fail for the docs has been fixed on master.
Merging :)
This PR adds a decorator that updates the dataset metadata if a previously executed transform modifies its features. 
Fixes #2083 
