List examples of custom data loaders

[jhamman]

This issue is meant to catalog existing Tensoflow and PyTorch data loaders that work with multi-dimensional data. The hope is that we can learn about some design patterns from existing loaders and build a generic solution that works with Xarray datasets.

Template:

Name: <short title>
ML library: >Tensorflow or PyTorch or other>
Link: <GitHub or other>
Description: <longer description, include example usage or pseudo code if possible>