Fix modules ignoring default device when created via native P/Invoke (#1438)

MoveModule<T>() was returning modules unchanged when device=null, instead
of resolving to torch.get_default_device(). This caused Embedding,
EmbeddingBag, LSTM, GRU, RNN, LSTMCell, GRUCell, RNNCell, and PReLU to
always be created on CPU regardless of the default device setting.

Fix MoveModule to use GetDefaultDeviceAndType() to resolve defaults,
aligning behavior with managed modules like Linear and Conv2d.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: alinpahontu2912

src/TorchSharp/NN/Module.cs

@@ -1138,10 +1138,8 @@ protected static (Device device, ScalarType dtype) GetDefaultDeviceAndType(Devic
                 internal T MoveModule<T>(Device? device, ScalarType? dtype) where T : Module
                 {
                     T module = (T)this;
-
-                    return device != null ?
-                       (dtype.HasValue ? (T)module._to(device, dtype.Value, false) : (T)module._to(device.type, device.index, false)) :
-                       (dtype.HasValue ? (T)module._to(dtype.Value, false) : module);
+                    var (targetDevice, targetDtype) = GetDefaultDeviceAndType(device, dtype);
+                    return (T)module._to(targetDevice, targetDtype, false);
                 }
 
                 protected void ClearModules() { _internal_submodules.clear(); }

test/TorchSharpTest/NN.cs

@@ -5264,6 +5264,37 @@ public void TestEmbeddingBagFromPretrained()
             }
         }
 
+        [Fact]
+        public void TestEmbeddingDefaultDevice()
+        {
+            // Regression test for https://github.com/dotnet/TorchSharp/issues/1438
+            // Embedding (and other native modules) ignored torch.set_default_device(),
+            // causing device mismatch when called with tensors on the default device.
+            var defaultDevice = torch.get_default_device();
+
+            try {
+                torch.set_default_device(torch.META);
+
+                // Before the fix, these modules were created on CPU despite META being
+                // the default device, which would cause "Expected all tensors to be on
+                // the same device" errors when used with tensors on the default device.
+                using (var emb = Embedding(2, 3)) {
+                    Assert.Equal(DeviceType.META, emb.weight!.device_type);
+                }
+
+                using (var emb = EmbeddingBag(2, 3)) {
+                    Assert.Equal(DeviceType.META, emb.weight!.device_type);
+                }
+
+                // Verify explicit device still takes precedence over default
+                using (var emb = Embedding(2, 3, device: torch.CPU)) {
+                    Assert.Equal(DeviceType.CPU, emb.weight!.device_type);
+                }
+            } finally {
+                torch.set_default_device(defaultDevice);
+            }
+        }
+
         [Fact]
         public void TestOneHotEncoding1()
         {