[models,configs/models] Cleanup model implementation to re-use all backbones from torchvision; Simplify checkpointer; Remove own implementation of model_zoo; Implement normalization for torchvision-based backbones; Enables pytorch installation on osx

[engine.*trainer] Set non-blocking operation for CPU->GPU data transfers to make communication asynchronous

[script,engine,test] Allows the user to specify the number of steps for threshold/performance analysis

[utils.metric] Rename as measure as some of the values are not real metrics; Add tests for AUC and improve its implementation

[utils.plot] Implement a more fair std.dev. plot using a bivariate gaussian modelling instead of vector addition (thinner regions)