Merge nucleic/sleek-ember-seal-uady into dev

tests/test_train_mlx.py

@@ -15,6 +15,38 @@ import train
 import train_mlx
 
 
+class MLXDeviceTests(unittest.TestCase):
+    class Metal:
+        def __init__(self, available):
+            self.available = available
+
+        def is_available(self):
+            return self.available
+
+    class MLX:
+        cpu = "cpu"
+        gpu = "gpu"
+
+        def __init__(self, metal_available):
+            self.metal = MLXDeviceTests.Metal(metal_available)
+            self.selected = None
+
+        def set_default_device(self, device):
+            self.selected = device
+
+    def test_cpu_is_an_explicit_fallback(self):
+        mlx = self.MLX(metal_available=False)
+        train_mlx._configure_mlx_device(mlx, "cpu")
+        self.assertEqual("cpu", mlx.selected)
+
+    def test_metal_fails_closed_when_unavailable(self):
+        with self.assertRaisesRegex(train.DataError, "requires Apple Silicon"):
+            train_mlx._configure_mlx_device(
+                self.MLX(metal_available=False),
+                "metal",
+            )
+
+
 class FixedShapeTokenizerTests(unittest.TestCase):
     class Tokenizer:
         pad_token_id = 0