Refactoring dataset module

[itsnamgyu]

Thank you for sharing the code for the CDFSL benchmark.

I'm currently working on a research project based on the code you've provided, but I noticed that there is a lot of duplicate code in the data and datasets folders. This makes it harder to modify the code, e.g., to customize the transformations across all the datasets.

Would you consider accepting a PR if I work on some refactorizations to remove these duplicates?

Thanks.