fix(autocast): gate autocast on AMP-capable devices

Add get_safe_autocast_context helper that only enters torch.autocast on
devices supporting AMP (cuda, xpu, cpu) and falls back to a no-op on mps
and unknown backends. Route the previously unconditional/underspecified
autocasts (vla_jepa, groot, molmoact2, lerobot_eval) through it so
autocast can be requested unconditionally without breaking on unsupported
devices.

tests/utils/test_device_utils.py

@@ -0,0 +1,40 @@
+# Copyright 2024 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 contextlib import nullcontext
+
+import pytest
+import torch
+
+from lerobot.utils.device_utils import get_safe_autocast_context
+
+
+@pytest.mark.parametrize(
+    ("device", "enabled", "expect_autocast"),
+    [
+        ("cpu", True, True),  # AMP-capable device -> real autocast
+        (torch.device("cpu"), True, True),  # accepts torch.device
+        ("cpu", False, False),  # explicitly disabled -> no-op
+        ("mps", True, False),  # AMP unsupported on mps -> no-op
+        ("privateuseone", True, False),  # unknown device -> safe no-op
+    ],
+)
+def test_get_safe_autocast_context(device, enabled, expect_autocast):
+    ctx = get_safe_autocast_context(device, dtype=torch.bfloat16, enabled=enabled)
+    if expect_autocast:
+        assert isinstance(ctx, torch.autocast)
+        with ctx:
+            assert torch.is_autocast_enabled("cpu")
+    else:
+        assert isinstance(ctx, nullcontext)