Last fixes for datasets' push_to_hub method

[LysandreJik]

This PR implements a few fixes necessary to have the datasets' push_to_hub method implemented:

• The regex for filenames that can be pushed to the hub does not allow the use of the - symbol which is necessary for files in the format <split_name>-00001-of-00010
• The list_repo_files actually only works for models. This adds support for datasets, and a future PR should also enable spaces.

[sgugger]

Thanks for fixing!

[lhoestq]

Thanks :)