Pre-trained model loading API

[aaarrti]

Description of the problem

• Quantus provides examples of simple models to get started with, e.g. https://github.com/understandable-machine-intelligence-lab/Quantus/blob/main/quantus/helpers/model/models.py#L128
• It is however up to user to download the weights from GitHub or event train them

Description of a solution

• Provide API similar to torchvision or keras.application, e.g.

import quantus

model = quantus.sample_models.LeNet3D(weights="MNITS")

Which allows the user to directly execute examples.

• Some weights are already out there, but probably we will need to train a few simple models, store in GitHub.
• Weights, that can be downloaded from raw GitHub content under the hood and cached in local file system.
• Provide clear description in readthedocs, which weights for which models are available.