Fix cupy error in KernelDensity with epanechnikov

[jcrist]

This works around an upstream bug in cupy where cupy.fuse sometimes fails to properly compile when executing operations on scalars. It's unclear to me why some scalar operations succeed while others fail, but as is epanechnikov was the only kernel that ran into this issue. Rewriting it slightly to work around the problem for now.

Fixes the nightly test failure for test_kernel_density
Fixes #6173.

[jcrist]

Hypothesis tests:

$ HYPOTHESIS_ENABLED=true py.test python/cuml/cuml/tests/test_kernel_density.py -k test_kernel_density -vv --pdb -s
========================================== test session starts ===========================================
platform linux -- Python 3.12.9, pytest-7.4.4, pluggy-1.5.0 -- /raid/jcristharif/miniforge3/envs/cuml-dev/bin/python3.12
cachedir: .pytest_cache
benchmark: 5.1.0 (defaults: timer=time.perf_counter disable_gc=False min_rounds=5 min_time=0.000005 max_time=1.0 calibration_precision=10 warmup=False warmup_iterations=100000)
hypothesis profile 'unit' -> max_examples=20, deadline=timedelta(milliseconds=2000), suppress_health_check=[HealthCheck.data_too_large, HealthCheck.too_slow], phases=(Phase.explicit, Phase.reuse, Phase.generate, Phase.target, Phase.shrink), database=DirectoryBasedExampleDatabase(PosixPath('/raid/jcristharif/cuml/.hypothesis/examples'))
rootdir: /raid/jcristharif/cuml/python/cuml
configfile: pyproject.toml
plugins: cov-6.0.0, cases-3.8.6, benchmark-5.1.0, xdist-3.6.1, hypothesis-6.125.1
collected 5 items

python/cuml/cuml/tests/test_kernel_density.py::test_kernel_density PASSED
python/cuml/cuml/tests/test_kernel_density.py::test_logaddexp PASSED
python/cuml/cuml/tests/test_kernel_density.py::test_metric_params PASSED
python/cuml/cuml/tests/test_kernel_density.py::test_grid_search PASSED
python/cuml/cuml/tests/test_kernel_density.py::test_not_fitted PASSED

[jcrist]

/merge