Use same `num_proc` for dataset download and generation
_The documentation is not available anymore as the PR was closed or merged._ I noticed this bug the other day and was going to look into it! "Where are these processes coming from?" ;-)
Use the same `num_proc` value for data download and generation. Additionally, do not set `num_proc` to 16 in `DownloadManager` by default (`num_proc` now has to be specified explicitly).