Function to vertically partition data

[TTitcombe]

Feature Description

A function which takes a torch dataset, e.g. torchvision.datasets.MNIST and outputs two datasets, each holding a deterministic partition of the dataset (i.e. the first returned dataset always corresponds to the top half of the images)

The datapoints should remain ordered (i.e. so PSI is not required)

Is your feature request related to a problem?

To provide vertically partitioned data to the SplitNN, we need an efficient and easy way to partition datasets

Additional Context

This is part of EPIC #3