Using `Dataset.map` with `n_proc>1` print multiple progress bars
Yes it allows to monitor the speed of each process. Currently each process takes care of one shard of the dataset.

At one point we can consider using streaming batches to a pool of processes instead of sharding the dataset in `num_proc` parts. At that point it will be easy to use only one progress bar
When using `Dataset.map` with `n_proc > 1`, only one of the processes should print a progress bar (to make the output readable). Right now, `n_proc` progress bars are printed.