Add demo data for multi-table scenario

[MooooCat]

Description

Motivation and Context

How has this been tested?

Types of changes

• Maintenance (no change in code, maintain the project's CI, docs, etc.)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[sweep-ai]

Apply Sweep Rules to your PR?

• Apply: All new business logic should have corresponding unit tests.
• Apply: Refactor large functions to be more modular.
• Apply: Add docstrings to all functions and file headers.

[MooooCat]

At present, the code of demo data is located in utils.py. I found that importing metadata in the implementation of utils.py will lead to circular import.

I recommend moving these implementations to a new file, such as dataset/demo.py, to make the functions of each module more clear.

What do you think of this solution? @Wh1isper

[codecov-commenter]

Codecov Report

Attention: 24 lines in your changes are missing coverage. Please review.

Comparison is base (c10270f) 79.32% compared to head (19f21b1) 78.59%.

Files	Patch %	Lines
sdgx/utils.py	14.28%	24 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #98      +/-   ##
==========================================
- Coverage   79.32%   78.59%   -0.74%     
==========================================
  Files          59       59              
  Lines        2496     2523      +27     
==========================================
+ Hits         1980     1983       +3     
- Misses        516      540      +24     

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[Wh1isper]

Moving demo Metadata into a single file will be better. This represents the user's preprocessing, not our utils.

[MooooCat]

Moving demo Metadata into a single file will be better. This represents the user's preprocessing, not our utils.

It makes sense, building metadata belongs to user process.

The function get_demo_multi_table will return multi DataFrame like get_demo_single_table, without Metadata.

[Wh1isper]

LGTM, thanks