Import handling improvements:

Motivation

This pull request refactors the import logic for scipy in optuna/_gp/scipy_blas_thread_patch.py to improve performance and avoid unnecessary imports. Instead of always importing scipy, it now uses a lazy import mechanism unless type checking is active.

Description of the changes

• Replaced the unconditional import of scipy with a conditional lazy import using _LazyImport, which helps defer loading scipy until it is actually needed.