[models.config] Make preset configurations location independent

src/ptbench/configs/models/__init__.py

src/ptbench/models/config/__init__.py

src/ptbench/models/config/alexnet.py

src/ptbench/models/config/alexnet_pretrained.py

src/ptbench/models/config/densenet.py

src/ptbench/models/config/densenet_pretrained.py

src/ptbench/models/config/densenet_rs.py

src/ptbench/models/config/logistic_regression.py

src/ptbench/models/config/pasa.py

src/ptbench/models/config/signs_to_tb.py