fix(randn_tensor): compare device.type, not torch.device, when suppressing MPS info log (#13508)

* fix(randn_tensor): compare device.type, not torch.device to str, when suppressing MPS info log

When a CPU generator is passed with an MPS target, randn_tensor intentionally skips
the 'generator was on cpu, tensor will be moved to <device>' info log — MPS doesn't
support device-side generators, so the suggestion to create one on MPS would be
misleading. The guard was written as `if device != "mps"`, but a few lines
earlier `device` is coerced to a `torch.device` object, and
`torch.device("mps") == "mps"` is False (torch.device's __eq__ with a string
returns NotImplemented, falling back to identity — they're different types).

Result: the guard is effectively always True, so MPS users get the spurious log
whenever they pass a CPU generator — the opposite of the documented intent.

Fix: compare `device.type` (a str) against "mps". Added a regression test in
tests/others/test_utils.py that exercises both the MPS and non-MPS paths via
`assertLogs` on the diffusers logger.

* refactor: use CaptureLogger instead of assertLogs in randn_tensor test

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
Co-authored-by: YiYi Xu <yixu310@gmail.com>

src/diffusers/utils/torch_utils.py

@@ -173,7 +173,7 @@ def randn_tensor(
         gen_device_type = generator.device.type if not isinstance(generator, list) else generator[0].device.type
         if gen_device_type != device.type and gen_device_type == "cpu":
             rand_device = "cpu"
-            if device != "mps":
+            if device.type != "mps":
                 logger.info(
                     f"The passed generator was created on 'cpu' even though a tensor on {device} was expected."
                     f" Tensors will be created on 'cpu' and then moved to {device}. Note that one can probably"

tests/others/test_utils.py

@@ -247,6 +247,43 @@ class FourierFilterTester(unittest.TestCase):
             assert out.shape == x.shape
 
 
+class RandnTensorTester(unittest.TestCase):
+    """Tests for :func:`diffusers.utils.torch_utils.randn_tensor`."""
+
+    def test_mps_suppresses_cpu_generator_info_log(self):
+        import torch
+
+        from diffusers.utils import logging as diffusers_logging
+        from diffusers.utils import torch_utils
+
+        from ..testing_utils import CaptureLogger
+
+        gen = torch.Generator(device="cpu")
+        diffusers_logging.set_verbosity_info()
+
+        def _capture(target_device):
+            with CaptureLogger(torch_utils.logger) as cl:
+                try:
+                    torch_utils.randn_tensor((1, 2), generator=gen, device=target_device, dtype=torch.float32)
+                except (AssertionError, RuntimeError):
+                    pass
+            return cl.out
+
+        mps_out = _capture("mps")
+        self.assertNotIn(
+            "moved to",
+            mps_out,
+            f"MPS target should not emit the CPU-fallback info log, got: {mps_out}",
+        )
+
+        cuda_out = _capture("cuda")
+        self.assertIn(
+            "moved to",
+            cuda_out,
+            f"Non-MPS target should still emit the CPU-fallback info log, got: {cuda_out}",
+        )
+
+
 # Copied from https://github.com/huggingface/transformers/blob/main/tests/utils/test_expectations.py
 class ExpectationsTester(unittest.TestCase):
     def test_expectations(self):