chore: lazy heavy imports (draft)

[nabinchha]

Strategy:
Create two facades as proxy for modules that are heavy

• src/data_designer/engine/__init__.py is a facade for all engine related public resources.
• src/data_designer/lazy_imports.py is a facade for external deps like pandas, pyarrow, etc.

Use resources through the facade:

from data_designer import lazy_imports

lazy_imports.pd.read_parquet(...)

from data_designer import engine

engine.ArtifactStorage.mkdir_if_needed(self._artifact_path)

Before after show roughly 92% load time improvement down to 1.6s from 18.3s.

may be we can name lazy_imports > lazy_third_party_imports but that's long.

We can change the usage above to be like the current pattern, but it has a slight perf hit (~2.8s):

Use resources through the facade:

from data_designer.lazy_imports import pd

pd.read_parquet(...)

from data_designer.engine import ArtifactStorage

ArtifactStorage.mkdir_if_needed(self._artifact_path)