add test to ensure ModelListGP works with only noise and no mask

test/models/test_model_list_gp_regression.py

@@ -479,6 +479,27 @@ def test_fantasize(self):
                     (3, 2), 0.3, dtype=x1.dtype, device=x1.device
                 )
                 observation_noise[:, 1] = 0.4
+
+                # check observation noise without mask
+                fm = modellist.fantasize(
+                    torch.rand(3, 2),
+                    sampler=ListSampler(sampler1, sampler2),
+                    observation_noise=observation_noise,
+                )
+                for i in range(2):
+                    fm_i = fm.models[i]
+                    self.assertIsInstance(fm_i, SingleTaskGP)
+                    self.assertIsInstance(fm_i.likelihood, FixedNoiseGaussianLikelihood)
+                    self.assertEqual(fm_i.train_inputs[0].shape, torch.Size([2, 8, 2]))
+                    self.assertEqual(fm_i.train_targets.shape, torch.Size([2, 8]))
+                    # check observation_noise
+                    self.assertTrue(
+                        torch.equal(
+                            fm_i.likelihood.noise[..., -3:], observation_noise[:, i]
+                        )
+                    )
+
+                # check masked noise
                 for obs_noise in (None, observation_noise):
                     fm = modellist.fantasize(
                         torch.rand(3, 2),