Make generalised dataset splitter functions (PyTorch)

[TTitcombe]

Feature Description

• Create functions which split PyTorch datasets into separate datasets
• Should work for image and non-image datasets
• Functions should apply random IDs to datapoints
• Unit test the functions

"Splitting" in this context means to split input features into two separate datasets. For images, split them top/bottom (a further issue will look to extend this)

Is your feature request related to a problem?

We should provide utility code to make it easy for people to turn non-vertically federated datasets into vertically federated ones, for experimental purposes.

We currently have some code built for this task, but it is not generaliseable to a wide range of datasets

What alternatives have you considered?

• Don't provide generaliseable code: This is okay for initial experimentation, but PyVertical should be a widely useable package for VFL