Conflicting guidance on lazy heavy imports

Summary

STYLEGUIDE.md and lazy_heavy_imports.py give conflicting guidance on importing heavy third-party libraries. The two sources should be aligned to clarify the expected pattern to use when writing new code.

The style guide

STYLEGUIDE.md notes that large dependencies should be lazy-loaded by importing the library alias from the data_designer.lazy_heavy_imports module:

https://github.com/NVIDIA-NeMo/DataDesigner/blob/7c2c15e3acfa716fc9de86d9b05fc4793962d9ca/STYLEGUIDE.md?plain=1#L145-L147

STYLEGUIDE.md also explains that any newly added heavy dependencies (e.g., your_lib) should use that same pattern:

https://github.com/NVIDIA-NeMo/DataDesigner/blob/7c2c15e3acfa716fc9de86d9b05fc4793962d9ca/STYLEGUIDE.md?plain=1#L176

The module docstring

In lazy_heavy_imports.py, the docstring warns against the exact pattern that STYLEGUIDE.md recommended:

https://github.com/NVIDIA-NeMo/DataDesigner/blob/7c2c15e3acfa716fc9de86d9b05fc4793962d9ca/packages/data-designer-config/src/data_designer/lazy_heavy_imports.py#L19-L21

Proposed resolution

Align both documentation sources to the same pattern to avoid confusion about which method is preferable, i.e., either importing the library alias from lazy_heavy_imports:

from data_designer.lazy_heavy_imports import pd, np 

or importing the lazy_heavy_imports module and accessing its attributes at runtime:

import data_designer.lazy_heavy_imports as lazy

# ...when needed in the code:
df = lazy.pd.DataFrame(...)

Read the guidance around lines 145-147 and 176 in STYLEGUIDE.md, then compare it with the docstring around lines 19-21 of packages/data-designer-config/src/data_designer/lazy_heavy_imports.py. Determine which lazy-import pattern the project wants to recommend, update both sources consistently, and verify that the examples and warnings no longer conflict.