Refactor multimodal dataset object to use the dance abstract dataset class

              TODO in a future PR: refactor dataset object to use the dance abstract dataset class, see [`ClusteringDataset`](https://github.com/OmicsML/dance/blob/f3e0e83ef3dbf25339e1a6ffa10ffb18ac81d72d/dance/datasets/singlemodality.py#L235) for example

_Originally posted by @RemyLau in https://github.com/OmicsML/dance/pull/228#discussion_r1129447102_
            