Set base path to hub url for canonical datasets
If we agree to have data files in a dedicated directory "data/" then we should be fine. You're right we should not try to edit a dataset script from the repository directly, but from github, in order to avoid conflicts
This should allow canonical datasets to use relative paths to download data files from the Hub

cc @polinaeterna this will be useful if we have audio datasets that are canonical and for which you'd like to host data files