add `desc` in `map` for `DatasetDict` object
The CI error is unrelated to the PR, merging @lhoestq, can we release this feature if you guys are planning for any patch release for Datasets. It'll slow down [#11927](https://github.com/huggingface/transformers/pull/11927) otherwise :/  Sure definitely, having a discrepancy between Dataset.map and DatasetDict.map is an issue that we should fix and include in a patch release. Will do it in the coming days
`desc` in `map` currently only works with `Dataset` objects. This PR adds support for `DatasetDict` objects as well