Description
This PR improves type checking in the optuna.importance module by moving imports that are only used for typing under a TYPE_CHECKING guard.

Motivation

• Reduces unnecessary runtime imports.
• Ensures cleaner separation between runtime and type-only dependencies.
• Aligns with Optuna’s type-checking conventions (see Use TYPE_CHECKING if necessary #6029).

Changes

• Updated optuna/importance/__init__.py so that Callable, Study, and FrozenTrial are imported only when TYPE_CHECKING is enabled.

Notes

• No runtime behaviour changes observed.
• Verified with mypy, 'flake8' and pytest -- all checks pass locally for the change.