build multi task Semantic Change detection model

[samysung]

Change detection model works well for binary change detection on simple case like building class or tree. However, when we want to apply it to more complexe case like multiclass changedetection. In the 2020 paper Multitask Learning for Large-scale Semantic Change Detection (https://arxiv.org/abs/1810.08452), they use a semantic segmentation model to bring better representation of each class for multi class change detection. The goad here is to build a pyotrch lightning to fit the goal of the associated milestone.