Use same `num_proc` for dataset download and generation
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_5300). All of your documentation changes will be reflected on that endpoint. 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).