【Hackathon 9th Sprint No.7】docs: Add PyTorch to Paddle conversion design document-part (#377)

* docs: Add PyTorch to Paddle conversion design document

- Add comprehensive design document for torch_to_paddle conversion task
- Document sample structure analysis and file conversion strategies
- Design modular architecture with convert.py, test.py, and utils.py
- Include risk assessment and mitigation strategies
- Define implementation phases and success criteria

* docs: Emphasize CompilerBackend implementation and test_compiler integration

- Add core design principle: converter must implement GraphCompilerBackend
- Document TorchToPaddleBackend implementation details
- Emphasize reusing existing test_compiler workflow for ES evaluation
- Update test workflow to use test_compiler instead of separate test script
- Document ES metrics acquisition through existing analysis tools
- Update implementation phases to include Backend development

Author: Dayuxiaoshui