ENH: Add Python factory override test

Adds test to verify that FFT base filter instantiation is overridden to
favor VkFFT acceleration. ITK v5.3rc04 introduced key changes to ITK
Python module loading to support default constructor overrides through
the ITK object factory.

wrapping/test/CMakeLists.txt

@@ -0,0 +1,5 @@
+if(ITK_WRAP_PYTHON)
+  itk_python_add_test(NAME itkVkFFTInitFactoryPythonTest
+      COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/itkVkFFTInitFactoryTest.py
+  )
+endif()

wrapping/test/itkVkFFTInitFactoryTest.py

@@ -0,0 +1,84 @@
+# ==========================================================================
+#
+#   Copyright NumFOCUS
+#
+#   Licensed under the Apache License, Version 2.0 (the "License");
+#   you may not use this file except in compliance with the License.
+#   You may obtain a copy of the License at
+#
+#          http://www.apache.org/licenses/LICENSE-2.0.txt
+#
+#   Unless required by applicable law or agreed to in writing, software
+#   distributed under the License is distributed on an "AS IS" BASIS,
+#   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#   See the License for the specific language governing permissions and
+#   limitations under the License.
+#
+
+# Verify that object factory initialization succeeds on itk module loading
+# such that ITK VkFFT classes are instantiated as the default FFT image filter
+# implementation through the object factory backend
+
+import itk
+
+itk.auto_progress(2)
+
+real_type = itk.F
+dimension = 3
+real_image_type = itk.Image[real_type, dimension]
+complex_image_type = itk.Image[itk.complex[real_type], dimension]
+
+# Verify all FFT base filter types are instantiated with VkFFT accelerated backend
+image_filter_list = [
+    (
+        itk.ComplexToComplex1DFFTImageFilter[complex_image_type],
+        itk.VkComplexToComplex1DFFTImageFilter[complex_image_type],
+    ),
+    (
+        itk.ComplexToComplexFFTImageFilter[complex_image_type],
+        itk.VkComplexToComplexFFTImageFilter[complex_image_type],
+    ),
+    (
+        itk.HalfHermitianToRealInverseFFTImageFilter[
+            complex_image_type, real_image_type
+        ],
+        itk.VkHalfHermitianToRealInverseFFTImageFilter[complex_image_type],
+    ),
+    (
+        itk.Forward1DFFTImageFilter[real_image_type],
+        itk.VkForward1DFFTImageFilter[real_image_type],
+    ),
+    (
+        itk.ForwardFFTImageFilter[real_image_type, complex_image_type],
+        itk.VkForwardFFTImageFilter[real_image_type],
+    ),
+    (
+        itk.Inverse1DFFTImageFilter[complex_image_type],
+        itk.VkInverse1DFFTImageFilter[complex_image_type],
+    ),
+    (
+        itk.InverseFFTImageFilter[complex_image_type, real_image_type],
+        itk.VkInverseFFTImageFilter[complex_image_type],
+    ),
+    (
+        itk.RealToHalfHermitianForwardFFTImageFilter[
+            real_image_type, complex_image_type
+        ],
+        itk.VkRealToHalfHermitianForwardFFTImageFilter[real_image_type],
+    ),
+]
+
+for (base_filter_type, vk_filter_type) in image_filter_list:
+    # Instantiate through the ITK object factory
+    image_filter = base_filter_type.New()
+    assert image_filter is not None
+
+    try:
+        print(
+            f"Instantiated default FFT image filter backend {image_filter.GetNameOfClass()}"
+        )
+        # Verify object can be cast to ITK VkFFT filter type
+        vk_filter_type.cast(image_filter)
+    except RuntimeError as e:
+        print(f"ITK VkFFT filter was not instantiated as default backend!")
+        raise e