Inference script - save images to disk and get PSNR metric per image

[balthazarneveu]

Prioritized because we want to verify that:

• NAFNet trained on densoising peformances are bad when there's almost no noise!
• Stacked conv has better PSNR Export images to disk

This is just a tiny subset of the tasks in #9

[balthazarneveu]

Allows comparing various architectures

[balthazarneveu]