fix CUDAExtension exception failing building package on a CPU-only system

add CUDA sources via MANIFEST.in explicitly for a case when building sdist on a CPU-only system

MANIFEST.in

 include requirements.txt
 include pyproject.toml
 include LICENSE*
+recursive-include torch_discounted_cumsum *
 recursive-include doc/img *
 recursive-include tests *
 global-exclude *.pyc
\ No newline at end of file

setup.py

@@ -35,25 +35,17 @@ https://www.github.com/toshas/torch-discounted-cumsum
 """
 
 
-setup(
-    name='torch_discounted_cumsum',
-    version='1.0.2',
-    description='Fast discounted cumulative sums in PyTorch',
-    long_description=long_description,
-    long_description_content_type='text/markdown',
-    install_requires=requirements,
-    python_requires='>=3.6',
-    packages=find_packages(),
-    author='Anton Obukhov',
-    license='BSD',
-    url='https://www.github.com/toshas/torch-discounted-cumsum',
-    ext_modules=[
+def configure_extensions():
+    out = [
         CppExtension(
             'torch_discounted_cumsum_cpu',
             [
                 os.path.join('torch_discounted_cumsum', 'discounted_cumsum_cpu.cpp'),
             ],
-        ),
+        )
+    ]
+    try:
+        out.append(
             CUDAExtension(
                 'torch_discounted_cumsum_cuda',
                 [
@@ -61,7 +53,25 @@ setup(
                     os.path.join('torch_discounted_cumsum', 'discounted_cumsum_cuda_kernel.cu'),
                 ],
             )
-    ],
+        )
+    except Exception as e:
+        print(f'Failed to build CUDA extension, this part of the package will not work. Reason: {str(e)}')
+    return out
+
+
+setup(
+    name='torch_discounted_cumsum',
+    version='1.0.2',
+    description='Fast discounted cumulative sums in PyTorch',
+    long_description=long_description,
+    long_description_content_type='text/markdown',
+    install_requires=requirements,
+    python_requires='>=3.6',
+    packages=find_packages(),
+    author='Anton Obukhov',
+    license='BSD',
+    url='https://www.github.com/toshas/torch-discounted-cumsum',
+    ext_modules=configure_extensions(),
     cmdclass={
         'build_ext': BuildExtension
     },