feat: Support direct construction of tvm_ffi.Function from callables

Add `__init__` to the Cython `Function` class so users can write:

    fadd = tvm_ffi.Function(add)

instead of going through `tvm_ffi.convert_to_tvm_func`. The constructor:
- Validates that the argument is callable
- Copies the handle (with incref) if the input is already a Function
- Otherwise converts via `_convert_to_ffi_func_handle`

Also adds the corresponding type stub in `core.pyi` and a test in
`test_function.py`.

Re-lands the original change reverted in #406.

Author: junrushao

python/tvm_ffi/core.pyi

@@ -184,6 +184,7 @@ class DLTensorTestWrapper:
 def _dltensor_test_wrapper_c_dlpack_from_pyobject_as_intptr() -> int: ...
 
 class Function(Object):
+    def __init__(self, func: Callable[..., Any]) -> None: ...
     @property
     def release_gil(self) -> bool: ...
     @release_gil.setter

python/tvm_ffi/cython/function.pxi

@@ -889,6 +889,27 @@ cdef class Function(Object):
     def __cinit__(self) -> None:
         self.c_release_gil = _RELEASE_GIL_BY_DEFAULT
 
+    def __init__(self, func: Callable[..., Any]) -> None:
+        """Initialize a Function from a Python callable.
+
+        This constructor allows creating a `tvm_ffi.Function` directly
+        from a Python function or another `tvm_ffi.Function` instance.
+
+        Parameters
+        ----------
+        func : Callable[..., Any]
+            The Python callable to wrap.
+        """
+        cdef TVMFFIObjectHandle chandle = NULL
+        if not callable(func):
+            raise TypeError(f"func must be callable, got {type(func)}")
+        if isinstance(func, Function):
+            chandle = (<Object>func).chandle
+            TVMFFIObjectIncRef(chandle)
+        else:
+            _convert_to_ffi_func_handle(func, &chandle)
+        self.chandle = chandle
+
     property release_gil:
         """Whether calls release the Python GIL while executing."""
 

tests/python/test_function.py

@@ -153,6 +153,15 @@ def fapply(f: Any, *args: Any) -> Any:
     assert fapply(add, 1, 3.3) == 4.3
 
 
+def test_pyfunc_init() -> None:
+    def add(a: int, b: int) -> int:
+        return a + b
+
+    fadd = tvm_ffi.Function(add)
+    assert isinstance(fadd, tvm_ffi.Function)
+    assert fadd(1, 2) == 3
+
+
 def test_global_func() -> None:
     @tvm_ffi.register_global_func("mytest.echo")
     def echo(x: Any) -> Any: