fix: Register root utility submodules in _import_structure to resolve lazy loading AttributeError

Register root utility submodules (`max_logging`, `max_utils`, `pyconfig`, `maxdiffusion_utils`, `aot_cache`, `multihost_dataloading`, `train_utils`, `tpu_utils`, `checkpointing`, `common_types`) in `_import_structure` in `src/maxdiffusion/__init__.py`.

During `_LazyModule` lazy module resolution, submodules importing top-level utility modules like `max_logging` via `from maxdiffusion import max_logging` triggered an `AttributeError` because these utility submodules were not registered in `_import_structure`. Python caught this `AttributeError` during submodule loading and raised `ImportError: cannot import name 'FlaxAutoencoderKL' from 'maxdiffusion.models'`.

Adding these utility submodules to `_import_structure` resolves root attribute lookups lazily, allowing imports of `FlaxAutoencoderKL`, `FlaxUNet2DConditionModel`, and `BaseStableDiffusionCheckpointer` to succeed.

Fixes #453