CUDA: Make numba.cuda.tests.doc_examples.ffi a module to fix #8252

[gmarkall]

The fix adds __init__.py to make numba.cuda.tests.doc_examples.ffi a module. This is intended to resolve Issue #8252 by ensuring the directory is installed and therefore included in packaging.

I've built the package locally, and the ffi folder now appears in the package:

$ ls ~/tmp/numba-issue-8252-package/lib/python3.9/site-packages/numba/cuda/tests/doc_examples/ffi
functions.cu  __init__.py  __pycache__

Installing the package locally with (the last couple of lines are because dependency resolution seems not to occur if a local package is installed):

mamba create -n numba-issue-8252-fix python=3.9 numba/label/dev::llvmlite
conda activate numba-issue-8252-fix
mamba install /home/gmarkall/mambaforge/conda-bld/linux-64/numba-0.57.0dev0-np1.16py3.9heec6ddf_g133cfe86d_91.tar.bz2
mamba install numpy
mamba install nvidia::cuda-python=11.7.0

and then running tests gives:

$ NUMBA_CUDA_USE_NVIDIA_BINDING=1 python -m numba.runtests numba.cuda.tests.doc_examples -v
test_ex_grid_sync (numba.cuda.tests.doc_examples.test_cg.TestCooperativeGroups) ... ok
test_ex_cpu_gpu_compat (numba.cuda.tests.doc_examples.test_cpu_gpu_compat.TestCpuGpuCompat) ... ok
test_ex_linking_cu (numba.cuda.tests.doc_examples.test_ffi.TestFFI) ... ok
test_ex_laplace (numba.cuda.tests.doc_examples.test_laplace.TestLaplace) ... ok
test_ex_matmul (numba.cuda.tests.doc_examples.test_matmul.TestMatMul)
Test of matrix multiplication on various cases. ... ok
test_ex_montecarlo (numba.cuda.tests.doc_examples.test_montecarlo.TestMonteCarlo) ... ok
test_ex_3d_grid (numba.cuda.tests.doc_examples.test_random.TestRandom) ... ok
test_ex_reduction (numba.cuda.tests.doc_examples.test_reduction.TestReduction) ... ok
test_ex_sessionize (numba.cuda.tests.doc_examples.test_sessionize.TestSessionization) ... ok
test_ex_vecadd (numba.cuda.tests.doc_examples.test_vecadd.TestVecAdd) ... ok

----------------------------------------------------------------------
Ran 10 tests in 4.213s

OK

indicating that the FFI test ran and succeeded.

Fixes #8252.

[gmarkall]

gpuci run tests

[sklam]

BFID numba_smoketest_cuda_yaml_141

[sklam]

I've checked this manually to confirm that it fixes the module not found error