fix(utils): raise ValueError from get_safe_torch_device (#3992)

* fix(utils): raise ValueError from get_safe_torch_device

Bare asserts vanish under python -O and look like programmer bugs.
Convert unavailable CUDA/MPS/XPU requests into clear ValueErrors.

* style: combine nested with in device util tests (ruff)

---------

Co-authored-by: Steven Palma <imstevenpmwork@ieee.org>

src/lerobot/utils/device_utils.py

@@ -37,16 +37,25 @@ def auto_select_torch_device() -> torch.device:
 
 # TODO(Steven): Remove log. log shouldn't be an argument, this should be handled by the logger level
 def get_safe_torch_device(try_device: str, log: bool = False) -> torch.device:
-    """Given a string, return a torch.device with checks on whether the device is available."""
+    """Given a string, return a torch.device with checks on whether the device is available.
+
+    Raises:
+        ValueError: If the requested device family is known but not available on
+            this machine (``AssertionError`` was previously used and is easy to
+            mistake for a programmer bug under ``python -O`` where asserts vanish).
+    """
     try_device = str(try_device)
     if try_device.startswith("cuda"):
-        assert torch.cuda.is_available()
+        if not torch.cuda.is_available():
+            raise ValueError(f"Requested device {try_device!r} but CUDA is not available.")
         device = torch.device(try_device)
     elif try_device == "mps":
-        assert torch.backends.mps.is_available()
+        if not torch.backends.mps.is_available():
+            raise ValueError("Requested device 'mps' but MPS is not available.")
         device = torch.device("mps")
     elif try_device == "xpu":
-        assert torch.xpu.is_available()
+        if not torch.xpu.is_available():
+            raise ValueError("Requested device 'xpu' but XPU is not available.")
         device = torch.device("xpu")
     elif try_device == "cpu":
         device = torch.device("cpu")

tests/utils/test_device_utils.py

@@ -0,0 +1,36 @@
+#!/usr/bin/env python
+# Copyright 2026 The HuggingFace Inc. team. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from unittest.mock import patch
+
+import pytest
+import torch
+
+from lerobot.utils.device_utils import get_safe_torch_device, is_torch_device_available
+
+
+def test_cpu_always_available():
+    assert get_safe_torch_device("cpu") == torch.device("cpu")
+    assert is_torch_device_available("cpu")
+
+
+def test_missing_cuda_raises_valueerror():
+    with patch("torch.cuda.is_available", return_value=False), pytest.raises(ValueError, match="CUDA"):
+        get_safe_torch_device("cuda")
+
+
+def test_missing_mps_raises_valueerror():
+    with patch("torch.backends.mps.is_available", return_value=False), pytest.raises(ValueError, match="MPS"):
+        get_safe_torch_device("mps")