feat(examples): refine flow examples with clean import structure

[keli-wen]

Description

This PR refines the flow examples structure by adding __init__.py files to each customized flow directory, enabling cleaner and more modular imports for major pipelines. #36

Key Changes

• Added __init__.py files to flow directories:

  • examples/flow/01_custom_flow/flows/greeting_flow/__init__.py - exports GreetingFlow and GreetingFlowConfig
  • examples/flow/02_summary_flow/flows/summary_flow/__init__.py - exports DemoSummaryFlow

• Updated import structure in pipeline files:

  • Changed from from flows.greeting_flow.flow import GreetingFlow to from flows.greeting_flow import GreetingFlow

• Enhanced documentation across all README files:

  • Added "Easy Import Structure" sections with import examples
  • Updated directory structure diagrams to show __init__.py files
  • Added new architecture principle about clean imports

Benefits

• Cleaner Imports: Enable directory-based imports like from flows.greeting_flow import GreetingFlow
• Better Modularity: Each flow directory now acts as a proper Python module
• Easier Integration: Major pipelines can more easily import and use custom flows
• Consistent Structure: All flow examples now follow the same modular pattern

Before/After Import Comparison

Before:

from flows.greeting_flow.flow import GreetingFlow, GreetingFlowConfig

After:

from flows.greeting_flow import GreetingFlow, GreetingFlowConfig

This change makes the flow system more intuitive and follows Python packaging best practices.

Checklist

• The PR title starts with feat(examples): refine flow examples with clean import structure
• Related issue is addressed (flow examples modularity improvement)
• The markdown and latex are rendered correctly
• The code in PR is well-documented with updated READMEs
• The PR is complete and focused on a single improvement (flow import structure)

[wanghaoxue0]

looks good from my side