Moved common functions to __init__ in parent directory

src/ptbench/data/__init__.py

src/ptbench/data/shenzhen/__init__.py

src/ptbench/data/shenzhen/default.py

src/ptbench/data/shenzhen/fold_0.py

src/ptbench/data/shenzhen/fold_0_rgb.py

src/ptbench/data/shenzhen/fold_1.py

src/ptbench/data/shenzhen/fold_1_rgb.py

src/ptbench/data/shenzhen/fold_2.py

src/ptbench/data/shenzhen/fold_2_rgb.py

src/ptbench/data/shenzhen/fold_3.py

src/ptbench/data/shenzhen/fold_3_rgb.py

src/ptbench/data/shenzhen/fold_4.py

src/ptbench/data/shenzhen/fold_4_rgb.py

src/ptbench/data/shenzhen/fold_5.py

src/ptbench/data/shenzhen/fold_5_rgb.py

src/ptbench/data/shenzhen/fold_6.py

src/ptbench/data/shenzhen/fold_6_rgb.py

src/ptbench/data/shenzhen/fold_7.py

src/ptbench/data/shenzhen/fold_7_rgb.py

src/ptbench/data/shenzhen/fold_8.py