doc: Improve python packaging doc

[junrushao]

Made a few changes:

• Migrate examples/packaging -> examples/python_packaging
• Add a prerequisite section
• Use literalinclude to include code from examples/python_packaging to the tutorial
• Point to Python package docs in tvm-ffi-stubgen helper messages

[gemini-code-assist]

Summary of Changes

Hello @junrushao, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly improves the Python packaging documentation by reorganizing example code, enhancing clarity with new sections and direct code inclusions, and updating the example C++ extension and its corresponding Python usage. These changes aim to provide a more accurate, maintainable, and user-friendly guide for developers working with TVM-FFI Python packaging.

Highlights

• Example Directory Migration: The examples/packaging directory has been renamed to examples/python_packaging to better reflect its content and purpose, ensuring a clearer structure for Python packaging examples.
• Documentation Enhancement: The python_packaging.rst documentation now includes a new 'Prerequisite' section and utilizes literalinclude directives to directly embed code snippets from the example files. This change ensures consistency between the documentation and the actual source code, making it easier to maintain and more accurate.
• Simplified C++ Extension Example: The extension.cc example has been refactored to be more concise, focusing on core FFI concepts like ABI-exported functions (AddTwo), global registry functions (AddOne, RaiseError), and simplified object definitions (IntPairObj with a Sum method). This removes TensorView examples and streamlines object usage.
• Updated Stub Generation Tool Help: The tvm-ffi-stubgen command-line interface help message has been streamlined. It now removes verbose examples and includes a direct link to the relevant Python packaging documentation, improving user guidance.
• Example Script Alignment: The run_example.py script and Python stub files (_ffi_api.py) have been updated to reflect the changes in the C++ extension. This includes new function calls, simplified object attribute access, and removal of the torch dependency from the example.

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[gemini-code-assist]

Code Review

This pull request primarily refactors and updates the Python packaging example for TVM-FFI. It deletes the old example.cc and introduces a new extension.cc which defines AddTwo (exported via TVM-FFI ABI), AddOne (registered globally), RaiseError, and an IntPairObj with a, b fields and a Sum method. The documentation (docs/packaging/python_packaging.rst) is updated to use literalinclude directives to pull code snippets directly from these example files, along with adding prerequisites and a note about the example code location. The CMakeLists.txt and pyproject.toml files in the example are updated with markers and explicit wheel configuration. The README.md is adjusted to reflect the new example structure and the four demonstration flows. Python stub files (_ffi_api.py) are updated to reflect the new add_one function and the IntPair object's sum method, replacing previous methods. Finally, the run_example.py script is modified to call the new add_two function, update add_one to a simpler integer operation, and correctly interact with the revised IntPair object. The tvm-ffi-stubgen CLI tool's help text is also simplified, pointing to external documentation.