ENH: support CuPy to_device "cpu"

array_api_compat/common/_helpers.py

@@ -154,6 +154,11 @@ def _cupy_to_device(x, device, /, stream=None):
 
     if device == x.device:
         return x
+    elif device == "cpu":
+        # allowing us to use `to_device(x, "cpu")`
+        # is useful for portable test swapping between
+        # host and device backends
+        return x.get()
     elif not isinstance(device, _Device):
         raise ValueError(f"Unsupported device {device!r}")
     else:

tests/test_common.py

@@ -0,0 +1,23 @@
+from ._helpers import import_
+from array_api_compat import to_device, device
+
+import pytest
+import numpy as np
+from numpy.testing import assert_allclose
+
+@pytest.mark.parametrize("library", ["cupy", "numpy", "torch"])
+def test_to_device_host(library):
+    # different libraries have different semantics
+    # for DtoH transfers; ensure that we support a portable
+    # shim for common array libs
+    # see: https://github.com/scipy/scipy/issues/18286#issuecomment-1527552919
+    xp = import_('array_api_compat.' + library)
+    expected = np.array([1, 2, 3])
+    x = xp.asarray([1, 2, 3])
+    x = to_device(x, "cpu")
+    # torch will return a genuine Device object, but
+    # the other libs will do something different with
+    # a `device(x)` query; however, what's really important
+    # here is that we can test portably after calling
+    # to_device(x, "cpu") to return to host
+    assert_allclose(x, expected)