Definition of a U net model using pytorch

[karlo-con-k]

Summary

This pull request introduces the implementation of the U-Net model for image segmentation tasks (using the paper 'U-Net: Convolutional Networks for Biomedical Image Segmentation' as reference), including:

A base ModelUNet class with a flexible architecture for different input sizes. A derived modeluNet class for smaller input sizes, with additional functionality to return intermediate outputs from the downsampling layers.