Address PR review comments for GELU approximate parameter

- Move Approximate enum from GELU module class to neutral
  TorchSharp namespace as GELUApproximate, removing Tensor/functional
  layer dependency on Modules layer
- Add CharSet, BestFitMapping, ThrowOnUnmappableChar attributes to
  THSTensor_gelu/gelu_ DllImport declarations to match existing
  LPStr-based imports pattern
- Update all references in Tensor.cs, GELU.cs, and tests

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

Author: alinpahontu2912

src/TorchSharp/NN/Activation/GELU.cs

@@ -14,22 +14,7 @@ namespace Modules
         /// </summary>
         public sealed class GELU : ParameterLessModule<Tensor, Tensor>
         {
-            /// <summary>
-            /// Specifies the approximation method for GELU.
-            /// </summary>
-            public enum Approximate
-            {
-                /// <summary>
-                /// Exact GELU computation.
-                /// </summary>
-                none,
-                /// <summary>
-                /// Tanh-based approximation.
-                /// </summary>
-                tanh
-            }
-
-            internal GELU(bool inplace, Approximate approximate = Approximate.none) : base(nameof(GELU))
+            internal GELU(bool inplace, GELUApproximate approximate = GELUApproximate.none) : base(nameof(GELU))
             {
                 this.inplace = inplace;
                 this.approximate = approximate;
@@ -42,7 +27,7 @@ public override Tensor forward(Tensor tensor)
 
             public bool inplace {get; set; }
 
-            public Approximate approximate { get; set; }
+            public GELUApproximate approximate { get; set; }
         }
     }
 
@@ -72,7 +57,7 @@ public static GELU GELU(bool inplace)
             /// </summary>
             /// <param name="approximate">The approximation method to use. Default: none</param>
             /// <param name="inplace">Do the operation in-place. Default: False</param>
-            public static GELU GELU(GELU.Approximate approximate, bool inplace = false)
+            public static GELU GELU(GELUApproximate approximate, bool inplace = false)
             {
                 return new GELU(inplace, approximate);
             }
@@ -95,7 +80,7 @@ public static Tensor gelu(Tensor x, bool inplace)
                 /// <param name="x">The input tensor</param>
                 /// <param name="approximate">The approximation method to use.</param>
                 /// <param name="inplace">Do the operation in-place. Default: False</param>
-                public static Tensor gelu(Tensor x, GELU.Approximate approximate, bool inplace = false)
+                public static Tensor gelu(Tensor x, GELUApproximate approximate, bool inplace = false)
                 {
                     return inplace ? x.gelu_(approximate).alias() : x.gelu(approximate);
                 }

src/TorchSharp/PInvoke/LibTorchSharp.THSTensor.cs

@@ -706,10 +706,10 @@ internal static extern IntPtr THSTensor_upsample_nearest3d(IntPtr input,
         [DllImport("LibTorchSharp")]
         internal static extern void THSTensor_elu_(IntPtr tensor, IntPtr alpha, IntPtr scale, IntPtr input_scale);
 
-        [DllImport("LibTorchSharp")]
+        [DllImport("LibTorchSharp", CharSet = CharSet.Ansi, BestFitMapping = false, ThrowOnUnmappableChar = true)]
         internal static extern IntPtr THSTensor_gelu(IntPtr tensor, [MarshalAs(UnmanagedType.LPStr)] string approximate);
 
-        [DllImport("LibTorchSharp")]
+        [DllImport("LibTorchSharp", CharSet = CharSet.Ansi, BestFitMapping = false, ThrowOnUnmappableChar = true)]
         internal static extern IntPtr THSTensor_gelu_(IntPtr tensor, [MarshalAs(UnmanagedType.LPStr)] string approximate);
 
         [DllImport("LibTorchSharp")]

src/TorchSharp/Tensor/Enums/GELUApproximate.cs

@@ -0,0 +1,18 @@
+// Copyright (c) .NET Foundation and Contributors.  All Rights Reserved.  See LICENSE in the project root for license information.
+namespace TorchSharp
+{
+    /// <summary>
+    /// Specifies the approximation method for the GELU activation function.
+    /// </summary>
+    public enum GELUApproximate
+    {
+        /// <summary>
+        /// Exact GELU computation.
+        /// </summary>
+        none,
+        /// <summary>
+        /// Tanh-based approximation.
+        /// </summary>
+        tanh
+    }
+}

src/TorchSharp/Tensor/Tensor.cs

@@ -2983,9 +2983,9 @@ public Tensor gelu()
                 return new Tensor(res);
             }
 
-            public Tensor gelu(TorchSharp.Modules.GELU.Approximate approximate)
+            public Tensor gelu(GELUApproximate approximate)
             {
-                var res = NativeMethods.THSTensor_gelu(Handle, approximate == TorchSharp.Modules.GELU.Approximate.tanh ? "tanh" : "none");
+                var res = NativeMethods.THSTensor_gelu(Handle, approximate == GELUApproximate.tanh ? "tanh" : "none");
                 if (res == IntPtr.Zero)
                     CheckForErrors();
                 return new Tensor(res);
@@ -2999,9 +2999,9 @@ public Tensor gelu_()
                 return new Tensor(res);
             }
 
-            public Tensor gelu_(TorchSharp.Modules.GELU.Approximate approximate)
+            public Tensor gelu_(GELUApproximate approximate)
             {
-                var res = NativeMethods.THSTensor_gelu_(Handle, approximate == TorchSharp.Modules.GELU.Approximate.tanh ? "tanh" : "none");
+                var res = NativeMethods.THSTensor_gelu_(Handle, approximate == GELUApproximate.tanh ? "tanh" : "none");
                 if (res == IntPtr.Zero)
                     CheckForErrors();
                 return new Tensor(res);

test/TorchSharpTest/NN.cs

@@ -621,7 +621,7 @@ public void EvaluateGELU()
         [Fact]
         public void EvaluateGELUWithTanhApproximate()
         {
-            var rel = GELU(Modules.GELU.Approximate.tanh);
+            var rel = GELU(GELUApproximate.tanh);
 
             foreach (var device in TestUtils.AvailableDevices()) {
                 var input = torch.randn(new long[] { 64, 8 }, device: device) * 25.0;
@@ -636,7 +636,7 @@ public void EvaluateGELUWithTanhApproximate()
             // Verify that tanh approximate produces different results from exact
             var x = torch.tensor(new float[] { -1.0f, 0.0f, 1.0f, 2.0f });
             var exact = torch.nn.functional.gelu(x);
-            var approx = torch.nn.functional.gelu(x, Modules.GELU.Approximate.tanh);
+            var approx = torch.nn.functional.gelu(x, GELUApproximate.tanh);
             Assert.False(exact.allclose(approx, rtol: 1e-5, atol: 1e-5));
         }