Compile API: output model and initializer stream write functions (#25455)

### Description
- Adds `ModelCompilationOptions_SetOutputModelWriteFunc` to the compile
API to allow writing the output model ONNX bytes to a user-provided
write function (i.e., for streaming).
- Adds `ModelCompilationOptions_SetOutputModelHandleInitializerFunc` to
the compile API to allow the user to write individual initializers to
some destination. Also allows specifying if an initializer should be
embedded within the ONNX model or written to a custom file.
- Adds C++, Python, and C# bindings for the new APIs.

A follow-up PR adds a write function for EPContext node binary data:
#25471

### Example
`ModelCompilationOptions_SetOutputModelWriteFunc`:
https://github.com/microsoft/onnxruntime/blob/c62ed23c328cbbfefd3083c1f7a6ced604772c19/onnxruntime/test/providers/qnn/qnn_ep_context_test.cc#L2075-L2131

`ModelCompilationOptions_SetOutputModelHandleInitializerFunc`:

https://github.com/microsoft/onnxruntime/blob/c62ed23c328cbbfefd3083c1f7a6ced604772c19/onnxruntime/test/providers/qnn/qnn_ep_context_test.cc#L2160-L2292

### Motivation and Context
Add output streaming capabilities when saving compiled models.

Author: adrianlizarraga

csharp/src/Microsoft.ML.OnnxRuntime/CompileModel.shared.cs

@@ -23,6 +23,8 @@ public struct OrtCompileApi
         public IntPtr ModelCompilationOptions_SetFlags;
         public IntPtr ModelCompilationOptions_SetEpContextBinaryInformation;
         public IntPtr ModelCompilationOptions_SetGraphOptimizationLevel;
+        public IntPtr ModelCompilationOptions_SetOutputModelWriteFunc;
+        public IntPtr ModelCompilationOptions_SetOutputModelGetInitializerLocationFunc;
     }
 
     internal class NativeMethods
@@ -118,6 +120,22 @@ public DOrtModelCompilationOptions_SetEpContextBinaryInformation
         public DOrtModelCompilationOptions_SetGraphOptimizationLevel
             OrtModelCompilationOptions_SetGraphOptimizationLevel;
 
+        [UnmanagedFunctionPointer(CallingConvention.Winapi)]
+        public delegate IntPtr /* OrtStatus* */ DOrtModelCompilationOptions_SetOutputModelWriteFunc(
+            IntPtr /* OrtModelCompilationOptions* */ options,
+            IntPtr /* DOrtWriteBufferDelegate */ writeFunc,
+            IntPtr /* void* */ state);
+        public DOrtModelCompilationOptions_SetOutputModelWriteFunc
+                        OrtModelCompilationOptions_SetOutputModelWriteFunc;
+
+        [UnmanagedFunctionPointer(CallingConvention.Winapi)]
+        public delegate IntPtr /* OrtStatus* */ DOrtModelCompilationOptions_SetOutputModelGetInitializerLocationFunc(
+            IntPtr /* OrtModelCompilationOptions* */ options,
+            IntPtr /* DOrtHandleInitializerDataDelegate */ handleInitializerFunc,
+            IntPtr /* void* */ state);
+        public DOrtModelCompilationOptions_SetOutputModelGetInitializerLocationFunc
+                        OrtModelCompilationOptions_SetOutputModelGetInitializerLocationFunc;
+
         internal NativeMethods(OnnxRuntime.NativeMethods.DOrtGetCompileApi getCompileApi)
         {
 
@@ -188,6 +206,17 @@ internal NativeMethods(OnnxRuntime.NativeMethods.DOrtGetCompileApi getCompileApi
                     _compileApi.ModelCompilationOptions_SetGraphOptimizationLevel,
                     typeof(DOrtModelCompilationOptions_SetGraphOptimizationLevel));
 
+            OrtModelCompilationOptions_SetOutputModelWriteFunc =
+                (DOrtModelCompilationOptions_SetOutputModelWriteFunc)Marshal.GetDelegateForFunctionPointer(
+                    _compileApi.ModelCompilationOptions_SetOutputModelWriteFunc,
+                    typeof(DOrtModelCompilationOptions_SetOutputModelWriteFunc));
+
+            OrtModelCompilationOptions_SetOutputModelGetInitializerLocationFunc =
+                (DOrtModelCompilationOptions_SetOutputModelGetInitializerLocationFunc)Marshal.
+                GetDelegateForFunctionPointer(
+                    _compileApi.ModelCompilationOptions_SetOutputModelGetInitializerLocationFunc,
+                    typeof(DOrtModelCompilationOptions_SetOutputModelGetInitializerLocationFunc));
+
         }
     }
 }

csharp/src/Microsoft.ML.OnnxRuntime/NativeCompileApiMethods.shared.cs

@@ -450,6 +450,7 @@ public struct OrtApi
 
         public IntPtr Graph_GetModelMetadata;
         public IntPtr GetModelCompatibilityForEpDevices;
+        public IntPtr CreateExternalInitializerInfo;
     }
 
     internal static class NativeMethods
@@ -787,9 +788,35 @@ static NativeMethods()
                     api_.SessionOptionsSetEpSelectionPolicyDelegate,
                     typeof(DSessionOptionsSetEpSelectionPolicyDelegate));
 
+            OrtReleaseExternalInitializerInfo =
+                (DOrtReleaseExternalInitializerInfo)Marshal.GetDelegateForFunctionPointer(
+                    api_.ReleaseExternalInitializerInfo,
+                    typeof(DOrtReleaseExternalInitializerInfo));
+
+            OrtExternalInitializerInfo_GetFilePath =
+                (DOrtExternalInitializerInfo_GetFilePath)Marshal.GetDelegateForFunctionPointer(
+                    api_.ExternalInitializerInfo_GetFilePath,
+                    typeof(DOrtExternalInitializerInfo_GetFilePath));
+
+            OrtExternalInitializerInfo_GetFileOffset =
+                (DOrtExternalInitializerInfo_GetFileOffset)Marshal.GetDelegateForFunctionPointer(
+                    api_.ExternalInitializerInfo_GetFileOffset,
+                    typeof(DOrtExternalInitializerInfo_GetFileOffset));
+
+            OrtExternalInitializerInfo_GetByteSize =
+                (DOrtExternalInitializerInfo_GetByteSize)Marshal.GetDelegateForFunctionPointer(
+                    api_.ExternalInitializerInfo_GetByteSize,
+                    typeof(DOrtExternalInitializerInfo_GetByteSize));
+
             OrtGetModelCompatibilityForEpDevices = (DOrtGetModelCompatibilityForEpDevices)Marshal.GetDelegateForFunctionPointer(
                 api_.GetModelCompatibilityForEpDevices,
                 typeof(DOrtGetModelCompatibilityForEpDevices));
+
+            OrtCreateExternalInitializerInfo =
+                (DOrtCreateExternalInitializerInfo)Marshal.GetDelegateForFunctionPointer(
+                    api_.CreateExternalInitializerInfo,
+                    typeof(DOrtCreateExternalInitializerInfo));
+
         }
 
         internal class NativeLib
@@ -2382,6 +2409,70 @@ out IntPtr lora_adapter
         public delegate ref CompileApi.OrtCompileApi DOrtGetCompileApi();
 #endif
         public static DOrtGetCompileApi OrtGetCompileApi;
+
+        /// <summary>
+        /// Delegate called by ORT to write a buffer (ONNX model bytes) to a custom destination (e.g., file or stream).
+        /// </summary>
+        /// <param name="state">State that was provided in when the delegate was registered.</param>
+        /// <param name="buffer">The buffer to write.</param>
+        /// <param name="bufferNumBytes">The size of the buffer in bytes.</param>
+        /// <returns>OrtStatus*</returns>
+        [UnmanagedFunctionPointer(CallingConvention.Winapi)]
+        public delegate IntPtr /* OrtStatus* */ DOrtWriteBufferToDestinationDelegate(
+            IntPtr /* void* */ state,
+            IntPtr /* const void* */ buffer,
+            UIntPtr /* size_t */ bufferNumBytes
+        );
+
+        /// <summary>
+        /// Function called by ORT to allow user to specify how an initializer should be saved while compiling
+        /// a model, that is, either written to an external file or stored within the model. ORT calls this function
+        /// for every initializer.
+        /// </summary>
+        /// <param name="state">State that was provided when the delegate was registered.</param>
+        /// <param name="initializerName">The initializer's name.</param>
+        /// <param name="initializerValue">The OrtValue containing the initializer's data, type, and shape</param>
+        /// <param name="externalInfo">The original initializer's location in an external file, or NULL.</param>
+        /// <param name="newExternalInfo">Output parameter set to a new OrtExternalInitializerInfo instance
+        /// indicating the location where the function implementation stored the initializer data. If the function
+        /// implementation sets `newExternalInfo` to NULL, ORT stores the initializer within the generated model.</param>
+        /// <returns></returns>
+        [UnmanagedFunctionPointer(CallingConvention.Winapi)]
+        public delegate IntPtr /* OrtStatus* */ DOrtGetInitializerLocationDelegate(
+            IntPtr /* void* */ state,
+            IntPtr /* const char* */ initializerName,
+            IntPtr /* const OrtValue* */ initializerValue,
+            IntPtr /* const OrtExternalInitializerInfo* */ externalInfo,
+            out IntPtr /* OrtExternalInitializerInfo** */ newExternalInfo
+        );
+
+        [UnmanagedFunctionPointer(CallingConvention.Winapi)]
+        public delegate void DOrtReleaseExternalInitializerInfo(IntPtr /* OrtExternalInitializerInfo* */ info);
+
+        [UnmanagedFunctionPointer(CallingConvention.Winapi)]
+        public delegate IntPtr /* OrtStatus* */ DOrtCreateExternalInitializerInfo(
+            byte[] /* const ORTCHAR_T* */ filePath,
+            long /* int64_t */ fileOffset,
+            UIntPtr /* size_t */ byteSize,
+            out IntPtr /* OrtExternalInitializerInfo** */ outInfo);
+
+        [UnmanagedFunctionPointer(CallingConvention.Winapi)]
+        public delegate IntPtr /* const ORTCHAR_T* */ DOrtExternalInitializerInfo_GetFilePath(
+            IntPtr /* const OrtExternalInitializerInfo* */ info);
+
+        [UnmanagedFunctionPointer(CallingConvention.Winapi)]
+        public delegate long /* int64_t */ DOrtExternalInitializerInfo_GetFileOffset(
+            IntPtr /* const OrtExternalInitializerInfo* */ info);
+
+        [UnmanagedFunctionPointer(CallingConvention.Winapi)]
+        public delegate UIntPtr /* size_t */ DOrtExternalInitializerInfo_GetByteSize(
+            IntPtr /* const OrtExternalInitializerInfo* */ info);
+
+        public static DOrtReleaseExternalInitializerInfo OrtReleaseExternalInitializerInfo;
+        public static DOrtCreateExternalInitializerInfo OrtCreateExternalInitializerInfo;
+        public static DOrtExternalInitializerInfo_GetFilePath OrtExternalInitializerInfo_GetFilePath;
+        public static DOrtExternalInitializerInfo_GetFileOffset OrtExternalInitializerInfo_GetFileOffset;
+        public static DOrtExternalInitializerInfo_GetByteSize OrtExternalInitializerInfo_GetByteSize;
 #endregion
 
 #region Auto EP API related

csharp/src/Microsoft.ML.OnnxRuntime/NativeMethods.shared.cs

@@ -150,6 +150,45 @@ internal static byte[] GetPlatformSerializedString(string str)
             else
                 return StringToZeroTerminatedUtf8(str);
         }
+
+        /// <summary>
+        /// Converts a null-terminated path string that is pointed to by the given IntPtr handle into
+        /// a C# UTF-16 string.
+        /// </summary>
+        /// <remarks>A path string on Windows is utf-16, but utf-8 on other operating systems.</remarks>
+        /// <param name="strPtr"></param>
+        /// <returns></returns>
+        internal static string StringFromNativePathString(IntPtr strPtr)
+        {
+            if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+            {
+                if (strPtr == IntPtr.Zero)
+                {
+                    return string.Empty;
+                }
+
+                // Get length of utf16 string by checking for two 0 bytes in a row.
+                int length = 0;
+                while (Marshal.ReadInt16(strPtr, length * 2) != 0)
+                {
+                    length += 1;
+                }
+
+                if (length == 0)
+                {
+                    return string.Empty;
+                }
+
+                unsafe
+                {
+                    return System.Text.Encoding.Unicode.GetString((byte*)strPtr, length * 2);
+                }
+            }
+            else
+            {
+                return StringFromNativeUtf8(strPtr);
+            }
+        }
     }
 
     // Guards an array of disposable objects on stack and disposes them in reverse order

csharp/src/Microsoft.ML.OnnxRuntime/NativeOnnxValueHelper.shared.cs

@@ -0,0 +1,136 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+
+namespace Microsoft.ML.OnnxRuntime
+{
+    using System;
+    using System.Diagnostics;
+    using System.Runtime.InteropServices;
+
+    /// <summary>
+    /// Class to that stores information about the file location where an "external" initializer is stored.
+    /// </summary>
+    /// <see cref="OrtModelCompilationOptions.HandleInitializerDelegate"/>
+    public class OrtExternalInitializerInfo : SafeHandle, IReadOnlyExternalInitializerInfo
+    {
+        // Set to false when constructed with an externally managed constant handle owned by ORT.
+        private readonly bool _ownsHandle = true;
+
+        /// <summary>
+        /// Create a new OrtExternalInitializerInfo instance. 
+        /// </summary>
+        /// <param name="filePath">The path to the file that stores the initializer data.</param>
+        /// <param name="fileOffset">The byte offset in the file where the data is stored.</param>
+        /// <param name="byteSize">The size of the data (in bytes) within the file.</param>
+        public OrtExternalInitializerInfo(string filePath, long fileOffset, long byteSize)
+            : base(IntPtr.Zero, ownsHandle: true)
+        {
+            var platformFilePath = NativeOnnxValueHelper.GetPlatformSerializedString(filePath);
+            NativeApiStatus.VerifySuccess(
+                NativeMethods.OrtCreateExternalInitializerInfo(platformFilePath, fileOffset, (UIntPtr)byteSize, out handle));
+            _ownsHandle = true;
+        }
+
+        /// <summary>
+        /// Create a new OrtExternalInitializerInfo instance from an existing native OrtExternalInitializerInfo handle.
+        /// </summary>
+        /// <param name="constHandle">Native OrtExternalInitializerInfo handle.</param>
+        /// <param name="ownsHandle">True if the OrtExternalInitializerInfo instance owns the native handle.
+        /// Defaults to false.</param>
+        internal OrtExternalInitializerInfo(IntPtr constHandle, bool ownsHandle = false)
+            : base(IntPtr.Zero, ownsHandle)
+        {
+            Debug.Assert(constHandle != IntPtr.Zero);
+            SetHandle(constHandle);
+            _ownsHandle = ownsHandle;
+        }
+
+        /// <summary>
+        /// Get the file path to the file that store's the initializer's data.
+        /// </summary>
+        /// <remarks>
+        /// The path is relative to the filesystem directory where the ONNX model was stored.
+        /// </remarks>
+        /// <returns>The file path.</returns>
+        public string GetFilePath()
+        {
+            IntPtr filePathPtr = NativeMethods.OrtExternalInitializerInfo_GetFilePath(handle);
+            if (filePathPtr == IntPtr.Zero)
+            {
+                return string.Empty;
+            }
+
+            return NativeOnnxValueHelper.StringFromNativePathString(filePathPtr);
+        }
+
+        /// <summary>
+        /// Get the byte offset within the file where the initializer's data is stored.
+        /// </summary>
+        /// <returns>The file offset location.</returns>
+        public long GetFileOffset()
+        {
+            return NativeMethods.OrtExternalInitializerInfo_GetFileOffset(handle);
+        }
+
+        /// <summary>
+        /// Get the size in bytes of the initializer's data within the file.
+        /// </summary>
+        /// <returns>The size in bytes of the initializer data.</returns>
+        public long GetByteSize()
+        {
+            UIntPtr byteSize = NativeMethods.OrtExternalInitializerInfo_GetByteSize(handle);
+            return checked((long)byteSize);
+        }
+
+        /// <summary>
+        /// Indicates whether the native handle is invalid.
+        /// </summary>
+        public override bool IsInvalid { get { return handle == IntPtr.Zero; } }
+
+        /// <summary>
+        /// Release the native instance of OrtExternalInitializerInfo if we own it.
+        /// </summary>
+        /// <returns>true on success and false on error.</returns>
+        protected override bool ReleaseHandle()
+        {
+            if (!_ownsHandle)
+            {
+                // Return false to indicate an error.
+                // ReleaseHandle() should not be called on a const handle that this class does not own.
+                return false;
+            }
+
+            NativeMethods.OrtReleaseExternalInitializerInfo(handle);
+            handle = IntPtr.Zero;
+            return true;
+        }
+    }
+
+    /// <summary>
+    /// Interface for all readonly methods implemented by OrtExternalInitializerInfo.
+    /// </summary>
+    public interface IReadOnlyExternalInitializerInfo
+    {
+        /// <summary>
+        /// Get the file path to the file that store's the initializer's data.
+        /// </summary>
+        /// <remarks>
+        /// The path is relative to the filesystem directory where the ONNX model was stored.
+        /// </remarks>
+        /// <returns>The file path.</returns>
+        string GetFilePath();
+
+        /// <summary>
+        /// Get the byte offset within the file where the initializer's data is stored.
+        /// </summary>
+        /// <returns>The file offset location.</returns>
+        long GetFileOffset();
+
+        /// <summary>
+        /// Get the size in bytes of the initializer's data within the file.
+        /// </summary>
+        /// <returns>The size in bytes of the initializer data.</returns>
+        long GetByteSize();
+    }
+}