test(losses): use self.skipTest instead of bare return for CUDA skip

Bare `return` in unittest does not signal a skip — pytest emits a
warning ("Did you mean to use assert instead of return?"). Replace
with `self.skipTest("CUDA not available")` which correctly marks the
test as skipped and is consistent with MONAI conventions.

Signed-off-by: Oleksandr Sanin <alexaaander.sanin@gmail.com>

Author: AlexanderSanin

tests/losses/image_dissimilarity/test_global_mutual_information_loss.py

@@ -181,7 +181,7 @@ def test_gaussian_buffers_move_with_module(self):
         self.assertEqual(loss.preterm.device.type, "cpu")
         self.assertEqual(loss.bin_centers.device.type, "cpu")
         if not torch.cuda.is_available():
-            return
+            self.skipTest("CUDA not available")
         loss = loss.cuda()
         self.assertEqual(loss.preterm.device.type, "cuda")
         self.assertEqual(loss.bin_centers.device.type, "cuda")