Resolve warning: AttributeError: '_Reduce' object has no attribute 'b…

…uild_result'

```
=========================================================================== warnings summary ===========================================================================
tests/test_reduce.py::test_reduce_non_contiguous
  /home/coder/cccl/python/devenv/lib/python3.12/site-packages/_pytest/unraisableexception.py:85: PytestUnraisableExceptionWarning: Exception ignored in: <function _Reduce.__del__ at 0x7bf123139080>

  Traceback (most recent call last):
    File "/home/coder/cccl/python/cuda_parallel/cuda/parallel/experimental/algorithms/reduce.py", line 132, in __del__
      bindings.cccl_device_reduce_cleanup(ctypes.byref(self.build_result))
                                                       ^^^^^^^^^^^^^^^^^
  AttributeError: '_Reduce' object has no attribute 'build_result'

    warnings.warn(pytest.PytestUnraisableExceptionWarning(msg))

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
============================================================= 1 passed, 93 deselected, 1 warning in 0.44s ==============================================================
```

python/cuda_parallel/cuda/parallel/experimental/algorithms/reduce.py

@@ -55,6 +55,9 @@ def __init__(
         op: Callable,
         h_init: np.ndarray,
     ):
+        # Referenced from __del__:
+        self.build_result = None
+
         d_in_cccl = cccl.to_cccl_iter(d_in)
         self._ctor_d_in_cccl_type_enum_name = cccl.type_enum_as_name(
             d_in_cccl.value_type.type.value
@@ -128,6 +131,8 @@ def __call__(self, temp_storage, d_in, d_out, num_items: int, h_init: np.ndarray
         return temp_storage_bytes.value
 
     def __del__(self):
+        if self.build_result is None:
+            return
         bindings = get_bindings()
         bindings.cccl_device_reduce_cleanup(ctypes.byref(self.build_result))