refactor: remove H5 storage backend in favor of mmap bin

- Remove H5Store, H5Writer, save_h5/load_h5 and h5py dependency
- MmapStore (bin) is the sole pre-tokenized storage backend
- Move setup_logging after imports to fix E402 in __init__.py
- Clean up unused imports across test files
- Move inline test imports to file top

tests/module/test_tie_weight.py

@@ -1,7 +1,6 @@
 import json
 import os
 
-import pytest
 import safetensors.torch as st
 import torch