feat(python): add Function.__init__ constructor for direct callable wrapping

[junrushao]

Summary

• Add Function.__init__ constructor to the Cython Function class, enabling direct construction via tvm_ffi.Function(callable) instead of requiring tvm_ffi.convert(callable)
• Support three input modes: wrap a plain Python callable, copy-construct from an existing Function (with ref-count increment), or construct with no arguments for backward compatibility (null handle)
• Reject non-callable inputs with TypeError and moved-from Function objects with ValueError

Architecture

• Adds __init__ to the Cython Function extension type in function.pxi. For plain callables, delegates to the existing _convert_to_ffi_func_handle helper. For Function-to-Function copies, increments the C-level ref count via TVMFFIObjectIncRef and shares the handle.
• func parameter defaults to None for backward compatibility: when omitted the object remains in its default null-handle state (matching the prior behavior where __cinit__ left chandle = NULL).
• No changes to the C ABI, C++ layer, or global registry.

Public Interfaces

• New: tvm_ffi.Function(func: Callable[..., Any] | None = None) -> Function constructor.
• Type stub updated in core.pyi.

Behavioral Changes

• Previously, tvm_ffi.Function could only be obtained via tvm_ffi.convert() or the global function registry. Now it can be constructed directly.
• Copy-constructing from a Function produces a second ref to the same underlying packed function (verified via same_as).
• Calling Function() with no arguments preserves backward compatibility by returning a null-handle instance identical to the prior default construction.

Tests

• test_pyfunc_init covers: wrapping a Python callable, copy-constructing from Function, ValueError on moved-from source, no-arg null-handle backward compat, TypeError on non-callable input.
• Moved-from test uses __new__ to avoid leaking a handle that would be immediately overwritten by __move_handle_from__.

Untested Edge Cases

• Thread-safety of concurrent __init__ calls (covered by GIL in CPython).
• Interaction with release_gil property on the newly constructed function (inherits default from __cinit__).

[gemini-code-assist]

Summary of Changes

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This pull request enhances the tvm_ffi.Function class by providing a direct constructor, simplifying the process of wrapping Python callables or duplicating existing Function objects. This change improves usability by offering a more intuitive initialization path, moving beyond reliance on tvm_ffi.convert() or the global function registry, while maintaining robust error handling for invalid inputs.

Highlights

• Direct Function Initialization: Introduced a Function.__init__ constructor in the Cython Function class, allowing direct creation from Python callables or existing Function instances.
• Input Handling: Implemented robust input validation, raising TypeError for non-callable inputs and ValueError for attempts to initialize from moved-from Function objects.
• Reference Counting: Ensured proper reference counting when copy-constructing from an existing Function by incrementing the C-level reference count.

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Changelog

• python/tvm_ffi/core.pyi
  • Added the __init__ method signature to the Function class.
• python/tvm_ffi/cython/function.pxi
  • Implemented the __init__ method for the Function Cython class.
  • Added logic to handle initialization from Python callables or existing Function instances.
  • Included error handling for non-callable inputs and moved-from Function objects.
  • Managed C-level reference counting for Function copies.
• tests/python/test_function.py
  • Added test_pyfunc_init to verify the new Function.__init__ constructor's behavior.
  • Included tests for wrapping callables, copy-construction, and error cases.

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[gemini-code-assist]

Code Review

This pull request introduces a convenient __init__ constructor for tvm_ffi.Function, allowing direct instantiation from callables. The implementation correctly handles both new callables and copy-construction from existing Function objects, including checks for moved-from instances. The accompanying tests are thorough and cover the main use cases and error conditions. I've found a minor memory leak in one of the new tests and suggested a fix. Overall, this is a great addition that improves the usability of the FFI.

[tqchen]

let us wait until cutedsl release cycles fixes the compact issue, the land this one