Refactor and improve test_ipw.py for better maintainability

Summary:
This diff significantly improves the IPW test file by:

1. **Enhanced Readability**: Renamed class from `Testipw` to `TestIPW` and improved variable names throughout
2. **Comprehensive Docstrings**: Added detailed docstrings to all test methods explaining their purpose and behavior
3. **Reduced Complexity**: Refactored complex test methods for better maintainability
4. **Better Test Structure**: Organized helper functions appropriately, positioning them before their first use
5. **Improved Test Logic**: Corrected test logic and validation throughout the file
6. **Fixed Edge Cases**: Enhanced error handling and validation tests

The overall file structure is now cleaner with better organization and documentation.

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Reviewed By: atavory

Differential Revision: D78629277

fbshipit-source-id: fb49b46778f7de631304455cea19e0a2ee3db102

Author: talgalili