Add dataset utils to load several commonly-used datasets

[loreloc]

I have a package datasets.py containing methods for loading several commonly-used datasets into ready-to-use Pytorch loaders:

• 20+ binary datasets;
• Image datasets from torchvision (e.g., MNIST family, CIFAR10);
• 5 datasets used in papers about normalizing flows;
• 6 2D artificial datasets.

[lkct]

Do we put it in examples (to help people learn how to use) or cirkit (as part of the lib)?

[arranger1044]

perhaps better in examples