bug[next]: Mark cupy tests as requires gpu (GridTools#1483)

It noticed we execute some tests that use cupy in the cpu CSCS-CI. These tests are always executed when cupy is installed (which usually means we have a gpu, but not necessarily). This PR changes this such that they are marked with `requires_gpu` and as such can be disabled.

tests/next_tests/unit_tests/embedded_tests/test_nd_array_field.py

@@ -34,7 +34,15 @@
 KDim = Dimension("KDim")
 
 
-@pytest.fixture(params=nd_array_field._nd_array_implementations)
+def nd_array_implementation_params():
+    for xp in nd_array_field._nd_array_implementations:
+        if hasattr(nd_array_field, "cp") and xp == nd_array_field.cp:
+            yield pytest.param(xp, id=xp.__name__, marks=pytest.mark.requires_gpu)
+        else:
+            yield pytest.param(xp, id=xp.__name__)
+
+
+@pytest.fixture(params=nd_array_implementation_params())
 def nd_array_implementation(request):
     yield request.param
 
@@ -272,12 +280,16 @@ def test_binary_operations_with_intersection(binary_arithmetic_op, dims, expecte
     assert np.allclose(op_result.ndarray, expected_result)
 
 
-@pytest.fixture(
-    params=itertools.product(
-        nd_array_field._nd_array_implementations, nd_array_field._nd_array_implementations
-    ),
-    ids=lambda param: f"{param[0].__name__}-{param[1].__name__}",
-)
+def product_nd_array_implementation_params():
+    for xp1 in nd_array_field._nd_array_implementations:
+        for xp2 in nd_array_field._nd_array_implementations:
+            marks = ()
+            if any(hasattr(nd_array_field, "cp") and xp == nd_array_field.cp for xp in (xp1, xp2)):
+                marks = pytest.mark.requires_gpu
+            yield pytest.param((xp1, xp2), id=f"{xp1.__name__}-{xp2.__name__}", marks=marks)
+
+
+@pytest.fixture(params=product_nd_array_implementation_params())
 def product_nd_array_implementation(request):
     yield request.param