Polygraphy: rename the shadowed is_on_gpu test so is_on_cpu is actually tested

[Anai-Guo]

Description

tools/Polygraphy/tests/util/test_array.py defines test_is_on_cpu twice:

@pytest.mark.parametrize("obj, is_on_cpu", [... True, True, False, False])
def test_is_on_cpu(obj, is_on_cpu):
    assert util.array.is_on_cpu(obj) == is_on_cpu      # line 86

@pytest.mark.parametrize("obj, is_on_gpu", [... False, False, True, True])
def test_is_on_cpu(obj, is_on_gpu):                    # line 99  <-- same name
    assert util.array.is_on_gpu(obj) == is_on_gpu

The second def rebinds the module-level name, so the first function object is
discarded before pytest ever sees it. The result is that util.array.is_on_cpu
has no test coverage at all — the four cases that pytest does collect under the
name test_is_on_cpu are in fact the is_on_gpu cases.

From the body and the parametrize ids it is clear the second one was meant to be
test_is_on_gpu (it asserts util.array.is_on_gpu, and its expectations are the
exact inverse of the first block's).

Reproduction

Minimal standalone reduction of the same two-def structure, before and after
this change:

$ pytest --collect-only -q test_before.py
test_before.py::test_is_on_cpu[np-False]
test_before.py::test_is_on_cpu[t_cpu-False]
test_before.py::test_is_on_cpu[t_cuda-True]
test_before.py::test_is_on_cpu[dev-True]
4 tests collected

$ pytest --collect-only -q test_after.py
test_after.py::test_is_on_cpu[np-True]
test_after.py::test_is_on_cpu[t_cpu-True]
test_after.py::test_is_on_cpu[t_cuda-False]
test_after.py::test_is_on_cpu[dev-False]
test_after.py::test_is_on_gpu[np-False]
test_after.py::test_is_on_gpu[t_cpu-False]
test_after.py::test_is_on_gpu[t_cuda-True]
test_after.py::test_is_on_gpu[dev-True]
8 tests collected

Note the ids in the "before" run: the surviving test_is_on_cpu carries the
is_on_gpu expectations (np-False), confirming the CPU test is gone.

Fix

Rename the second definition to test_is_on_gpu. One line, no behavioural
change to Polygraphy itself; it restores the four is_on_cpu cases
(4 collected → 8 collected in this file).

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