WIP: pylibcugraph: declare cupy and numpy hard dependencies

[jameslamb]

While testing stuff for #4804, I found that pylibcugraph has a hard runtime dependency on cupy and numpy, but isn't declaring them

docker run \
    --rm \
    --gpus 0 \
    -it rapidsai/ci-wheel:latest \
    bash

python -m pip install 'pylibcugraph-cu12==25.2.*,>=0.0.0a0'
python -c "import pylibcugraph"

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/pyenv/versions/3.12.7/lib/python3.12/site-packages/pylibcugraph/__init__.py", line 18, in <module>
    from pylibcugraph.graphs import SGGraph, MGGraph
  File "graphs.pyx", line 1, in init pylibcugraph.graphs
  File "utils.pyx", line 20, in init pylibcugraph.utils
ModuleNotFoundError: No module named 'cupy'

Notes for Reviewers

Evidence that pylibcugraph already has a hard dependency on these libraries

They're used unconditionally here:

cugraph/python/pylibcugraph/pylibcugraph/utils.pyx

Lines 19 to 20 in cddd69e

	import numpy
	import cupy

But have import guards in other places:

cugraph/python/pylibcugraph/pylibcugraph/sssp.pyx

Lines 127 to 139 in cddd69e

	# FIXME: import these modules here for now until a better pattern can be
	# used for optional imports (perhaps 'import_optional()' from cugraph), or
	# these are made hard dependencies.
	try:
	import cupy
	except ModuleNotFoundError:
	raise RuntimeError("sssp requires the cupy package, which could not "
	"be imported")
	try:
	import numpy
	except ModuleNotFoundError:
	raise RuntimeError("sssp requires the numpy package, which could not "
	"be imported")

So this PR doesn't introduce new hard dependencies... it just makes them explicit, to make it easier to install and run pylibcugraph.

How was this not caught in CI?

Import tests aren't run here for conda packages, because conda builds happen on CPU-only nodes.

cugraph/ci/build_python.sh

Lines 27 to 30 in cddd69e

	rapids-conda-retry mambabuild \
	--no-test \
	--channel "${CPP_CHANNEL}" \
	conda/recipes/pylibcugraph

And numpy and cupy are probably getting pulled in by some of the wheels' test dependencies, like cudf, here:

cugraph/ci/test_wheel.sh

Line 17 in cddd69e

python -m pip install $(ls ./dist/${python_package_name}*.whl)[test]

Should we just make the other unconditional cases conditional with try-catching?

No. Talked with @rlratzel, @ChuckHastings, and @eriknw offline, and we agreed to declare these as hard runtime dependencies (and remove the try-catching in places that had it).

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[jameslamb]

/ok to test