fix get_actual_allocator

src/gt4py/next/_allocators.py

@@ -6,8 +6,6 @@
 # Please, refer to the LICENSE file in the root directory.
 # SPDX-License-Identifier: BSD-3-Clause
 
-# TODO: make this module private
-
 import abc
 import dataclasses
 import functools
@@ -407,13 +405,12 @@ def _check_unsupported_device_and_aligned_index(
 def _get_actual_allocator_and_device(
     allocator: Optional[FieldBufferAllocationUtil], device: Optional[core_defs.Device]
 ) -> tuple[FieldBufferAllocatorProtocol, core_defs.Device]:
-    if device is None and allocator is None:
-        raise ValueError("No 'device' or 'allocator' specified.")
-    actual_allocator = get_allocator(allocator)
-    if actual_allocator is None:
-        assert device is not None  # for mypy
-        actual_allocator = device_allocators[device.device_type]
-    elif device is None:
+    if allocator is None and device is not None:
+        return device_allocators[device.device_type], device
+
+    actual_allocator = get_allocator(allocator, default=device_allocators[core_defs.DeviceType.CPU])
+    assert actual_allocator is not None
+    if device is None:
         device = core_defs.Device(actual_allocator.__gt_device_type__, 0)
     elif device.device_type != actual_allocator.__gt_device_type__:
         raise ValueError(f"Device '{device}' and allocator '{actual_allocator}' are incompatible.")