refactor: customize workflow example with configurable date range

[asika]

Description

Refactored the workflow_by_code.py example to make it more flexible and easier to customize. The main changes include:

1. Updated imports: Changed from importing CSI300_GBDT_TASK to importing CSI300_MARKET and GBDT_MODEL separately for better modularity
2. Introduced configurable date range: Added TEST_START and TEST_END variables to easily customize the prediction period (currently set to predict 10 trading days starting from 2017-01-01)
3. Inlined task configuration: Replaced the hardcoded CSI300_GBDT_TASK reference with an explicit task_config dictionary that clearly shows all configuration parameters
4. Updated backtest dates: Changed backtest period to use the configurable TEST_START and TEST_END variables instead of hardcoded dates

These changes make the example more transparent and easier for users to understand and modify the workflow configuration for their own use cases.

Motivation and Context

The original example relied on a pre-defined task configuration that was not visible in the code. By inlining the configuration and making key parameters (like date ranges) configurable, users can more easily understand what the example does and customize it for their needs.

How Has This Been Tested?

• Pass the test by running: pytest qlib/tests/test_all_pipeline.py under upper directory of qlib.
• If you are adding a new feature, test on your own test scripts.

Types of changes

• Update documentation
• Refactor code (improve clarity and maintainability)

https://claude.ai/code/session_0166zKhXb3BvVHad66b1ZKYw

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