fix: correct onnx field numbers and address pr review comments

CRITICAL: Fix ONNX TensorProto field number compliance:
- OnnxProto.cs: Change field 3 → 8 for tensor name per ONNX spec
- OnnxToCoreMLConverter.cs: Fix all TensorProto fields (1=dims, 2=data_type, 8=name, 9=raw_data)
- Previous incorrect field numbers would cause empty tensor names and broken shape inference

Additional fixes:
- CoreMLExporter.cs: Fix QuantizationBits mapping (Int8→8, Float16→16, default→32)
- TensorRTConfiguration.cs: Use ArgumentException instead of ArgumentNullException for whitespace validation
- ModelExporterBase.cs: Remove redundant null check (IsNullOrWhiteSpace handles null)

Addresses PR #486 review comments #1, #2, #4, #5, #6

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ooples

src/Deployment/Export/ModelExporterBase.cs

@@ -38,7 +38,7 @@ public virtual void Export(IFullModel<T, TInput, TOutput> model, string outputPa
         // Ensure output directory exists
         // Path.GetDirectoryName can return null for root paths or relative filenames
         var directory = Path.GetDirectoryName(outputPath);
-        if (directory is not null && !string.IsNullOrWhiteSpace(directory))
+        if (!string.IsNullOrWhiteSpace(directory))
         {
             if (!Directory.Exists(directory))
             {

src/Deployment/Export/Onnx/OnnxProto.cs

@@ -353,8 +353,8 @@ private static byte[] CreateTensorProto<T>(string name, Vector<T> data)
         writer.WriteTag(2, WireFormat.WireType.Varint);
         writer.WriteInt32(GetDataTypeValue(typeof(T)));
 
-        // Field 3: name
-        writer.WriteTag(3, WireFormat.WireType.LengthDelimited);
+        // Field 8: name (per ONNX TensorProto specification)
+        writer.WriteTag(8, WireFormat.WireType.LengthDelimited);
         writer.WriteString(name);
 
         // Field 9: raw_data

src/Deployment/Mobile/CoreML/CoreMLExporter.cs

@@ -84,7 +84,12 @@ private byte[] ConvertOnnxToCoreML(byte[] onnxBytes, ExportConfiguration config)
                 ModelName = config.ModelName,
                 ModelDescription = config.ModelDescription,
                 OptimizeForSize = true,
-                QuantizationBits = config.QuantizationMode == QuantizationMode.Float16 ? 16 : 32
+                QuantizationBits = config.QuantizationMode switch
+                {
+                    QuantizationMode.Int8 => 8,
+                    QuantizationMode.Float16 => 16,
+                    _ => 32
+                }
             };
         }
 

src/Deployment/Mobile/CoreML/OnnxToCoreMLConverter.cs

@@ -145,14 +145,14 @@ private static OnnxTensor ParseTensor(byte[] tensorBytes)
 
             switch (fieldNumber)
             {
-                case 3: // name
-                    name = reader.ReadString();
+                case 1: // dims (repeated) - ONNX TensorProto field 1
+                    dims.Add(reader.ReadInt64());
                     break;
-                case 5: // data_type
+                case 2: // data_type - ONNX TensorProto field 2
                     dataType = reader.ReadInt32();
                     break;
-                case 7: // dims (repeated)
-                    dims.Add(reader.ReadInt64());
+                case 8: // name - ONNX TensorProto field 8
+                    name = reader.ReadString();
                     break;
                 case 9: // raw_data
                     var rawBytes = reader.ReadBytes().ToByteArray();

src/Deployment/TensorRT/TensorRTConfiguration.cs

@@ -157,12 +157,11 @@ public static TensorRTConfiguration ForHighThroughput(int batchSize = 64, string
     /// Creates a configuration with INT8 quantization.
     /// </summary>
     /// <param name="calibrationDataPath">Path to calibration data file (required for INT8 quantization)</param>
-    /// <exception cref="ArgumentNullException">Thrown when calibrationDataPath is null or whitespace</exception>
+    /// <exception cref="ArgumentException">Thrown when calibrationDataPath is null or whitespace</exception>
     public static TensorRTConfiguration ForInt8(string calibrationDataPath)
     {
         if (string.IsNullOrWhiteSpace(calibrationDataPath))
-            throw new ArgumentNullException(nameof(calibrationDataPath),
-                "Calibration data path is required for INT8 quantization");
+            throw new ArgumentException("Calibration data path cannot be null or whitespace", nameof(calibrationDataPath));
 
         return new TensorRTConfiguration
         {