[examples] add desc to `dataset.map` to improve tqdm bars
https://github.com/huggingface/datasets/pull/2374 has been merged - we should deploy this feature in our examples, which would tell the user what's being processed and the tqdm bar is for.

Currently we get a bunch of bars that are absolutely meaningless and hard to understand what they do. See also: https://github.com/huggingface/datasets/issues/2330

The only issue is how to depend on `datasets` dev version, might have to wait for a new `datasets` release 1.6.3 to be able to merge such PR.

A new release should be made in the next few days I'm being told, so a PR can be made.