Reduce memory in `_get_graph`/`_get_graph_supervised` #3530

Job	Run time
check-nightly-ci	4s
telemetry-setup	3s
changed-files / Check changed files	21s
checks / other-checks	1m 24s
checks / check-style	1m 44s
devcontainer / amd64, 12.8, conda	14m 45s
devcontainer / amd64, 12.8, pip	28m 6s
conda-cpp-build	0s
wheel-build-libcuml	0s
clang-tidy	0s
conda-cpp-checks	0s
conda-cpp-tests	0s
conda-python-build	0s
wheel-build-cuml	0s
conda-python-tests-dask	0s
optional-job-conda-python-tests-cudf-pandas-integration	0s
conda-python-tests-singlegpu	0s
conda-notebook-tests	0s
docs-build	0s
wheel-tests-cuml	0s
pr-builder / run	1s
telemetry-summarize	22s
	46m 50s

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