[model] Model takes loss type and arguments during instanciation

src/mednet/config/models/alexnet.py

src/mednet/config/models/alexnet_pretrained.py

src/mednet/config/models/densenet.py

src/mednet/config/models/densenet_pretrained.py

src/mednet/config/models/densenet_rs.py

src/mednet/config/models/pasa.py

src/mednet/engine/trainer.py

src/mednet/models/alexnet.py

src/mednet/models/densenet.py

src/mednet/models/model.py

src/mednet/models/pasa.py