```python
Structure containing metadata from a RepoCard.

[`CardData`] is the parent class of [`ModelCardData`] and [`DatasetCardData`].

Metadata can be exported as a dictionary or YAML. Export can be customized to alter the representation of the data
(example: flatten evaluation results). `CardData` behaves as a dictionary (can get, pop, set values) but do not
inherit from `dict` to allow this export step.
```