Depracate `num_proc` parameter in `DownloadManager.extract`
I can take this! #self-assign #self-assign @lazarust i'm already working on this issue :smile:  #self-assign hey @mariosasko , i made a pr for this issue. Could you please review it.
The `num_proc` parameter is only present in `DownloadManager.extract` but not in `StreamingDownloadManager.extract`, making it impossible to support streaming in the dataset scripts that use it (`openwebtext` and `the_pile_stack_exchange`). We can avoid this situation by deprecating this parameter and passing `DownloadConfig`'s `num_proc` to `map_nested` instead, as it's done in `DownloadManager.download`.