Add GetCapability/Compile infrastructure for EP ABI (microsoft#24887)

### Description
This PRs sets the foundation for the EP ABI, which allows plugin-EPs to
interface with ORT using a binary stable interface. A plugin-EP can be
built separately from ORT and is not tied to a specific commit of ORT.

Currently, this PR adds basic APIs necessary to allow an example
plugin-EP to compile and run a simple model with a single `Mul` node.

- Example plugin-EP implementation:
https://github.com/microsoft/onnxruntime/blob/adrianl/ep-abi/onnxruntime/test/autoep/library/example_plugin_ep.cc
- APIs: 
- Graph IR:
https://github.com/microsoft/onnxruntime/blob/adrianl/ep-abi/include/onnxruntime/core/session/onnxruntime_c_api.h#L5290-L5439
- Plugin EP:
https://github.com/microsoft/onnxruntime/blob/adrianl/ep-abi/include/onnxruntime/core/session/onnxruntime_c_api.h#L6177-L6481
- Example app code (from unit tests):
https://github.com/microsoft/onnxruntime/blob/adrianl/ep-abi/onnxruntime/test/autoep/test_autoep_selection.cc#L614

### Motivation and Context
Based on microsoft#21450

---------

Co-authored-by: Copilot <[email protected]>
Co-authored-by: Edward Chen <[email protected]>

Author: 3 people

cmake/onnxruntime.cmake

@@ -22,6 +22,7 @@ endif()
 function(get_c_cxx_api_headers HEADERS_VAR)
   set(_headers
     "${REPO_ROOT}/include/onnxruntime/core/session/onnxruntime_c_api.h"
+    "${REPO_ROOT}/include/onnxruntime/core/session/onnxruntime_ep_c_api.h"
     "${REPO_ROOT}/include/onnxruntime/core/session/onnxruntime_cxx_api.h"
     "${REPO_ROOT}/include/onnxruntime/core/session/onnxruntime_cxx_inline.h"
     "${REPO_ROOT}/include/onnxruntime/core/session/onnxruntime_float16.h"

cmake/onnxruntime_unittests.cmake

@@ -495,6 +495,7 @@ set (ONNXRUNTIME_GLOBAL_THREAD_POOLS_TEST_SRC_DIR "${TEST_SRC_DIR}/global_thread
 set (ONNXRUNTIME_CUSTOM_OP_REGISTRATION_TEST_SRC_DIR "${TEST_SRC_DIR}/custom_op_registration")
 set (ONNXRUNTIME_LOGGING_APIS_TEST_SRC_DIR "${TEST_SRC_DIR}/logging_apis")
 set (ONNXRUNTIME_AUTOEP_TEST_SRC_DIR "${TEST_SRC_DIR}/autoep")
+set (ONNXRUNTIME_EP_GRAPH_TEST_SRC_DIR "${TEST_SRC_DIR}/ep_graph")
 
 set (onnxruntime_shared_lib_test_SRC
           ${ONNXRUNTIME_SHARED_LIB_TEST_SRC_DIR}/test_fixture.h
@@ -1848,8 +1849,8 @@ if (WIN32 AND onnxruntime_BUILD_SHARED_LIB AND
                ${ONNXRUNTIME_AUTOEP_LIB_LINK_FLAG})
 
   # test library
-  file(GLOB_RECURSE onnxruntime_autoep_test_SRC "${ONNXRUNTIME_AUTOEP_TEST_SRC_DIR}/*.h"
-                                                "${ONNXRUNTIME_AUTOEP_TEST_SRC_DIR}/*.cc")
+  file(GLOB onnxruntime_autoep_test_SRC "${ONNXRUNTIME_AUTOEP_TEST_SRC_DIR}/*.h"
+                                        "${ONNXRUNTIME_AUTOEP_TEST_SRC_DIR}/*.cc")
 
   set(onnxruntime_autoep_test_LIBS onnxruntime_mocked_allocator ${ONNXRUNTIME_TEST_LIBS} onnxruntime_test_utils
                                    onnx_proto onnx ${onnxruntime_EXTERNAL_LIBRARIES})
@@ -1988,4 +1989,34 @@ if (onnxruntime_USE_WEBGPU AND WIN32 AND onnxruntime_BUILD_SHARED_LIB AND NOT CM
   )
 endif()
 
+# onnxruntime_ep_graph_test tests the implementation of the public OrtGraph APIs for use in plugin EPs (OrtEp).
+if (onnxruntime_BUILD_SHARED_LIB AND NOT CMAKE_SYSTEM_NAME STREQUAL "Emscripten" AND NOT onnxruntime_MINIMAL_BUILD)
+  file(GLOB_RECURSE onnxruntime_ep_graph_test_SRC "${ONNXRUNTIME_EP_GRAPH_TEST_SRC_DIR}/*.h"
+                                                  "${ONNXRUNTIME_EP_GRAPH_TEST_SRC_DIR}/*.cc")
+
+  set(onnxruntime_ep_graph_test_LIBS ${ONNXRUNTIME_TEST_LIBS} onnxruntime_test_utils ${onnxruntime_EXTERNAL_LIBRARIES})
+  if (CMAKE_SYSTEM_NAME MATCHES "AIX")
+    list(APPEND onnxruntime_ep_graph_test_LIBS onnxruntime_session onnxruntime_util onnxruntime_lora onnxruntime_framework
+                                               onnxruntime_common onnxruntime_graph onnxruntime_providers onnxruntime_mlas
+                                               onnxruntime_optimizer onnxruntime_flatbuffers iconv re2
+                                               ${PROTOBUF_LIB} onnx onnx_proto)
+  endif()
+
+  if(NOT WIN32)
+    list(APPEND onnxruntime_ep_graph_test_LIBS  ${CMAKE_DL_LIBS})
+  endif()
+
+  if (onnxruntime_USE_TENSORRT OR onnxruntime_USE_NV)
+    # Need this because unittest_main_src defines a global nvinfer1::IBuilder variable.
+    list(APPEND onnxruntime_ep_graph_test_LIBS ${TENSORRT_LIBRARY_INFER})
+  endif()
+
+  AddTest(DYN
+          TARGET onnxruntime_ep_graph_test
+          SOURCES ${onnxruntime_ep_graph_test_SRC} ${onnxruntime_unittest_main_src}
+          LIBS ${onnxruntime_ep_graph_test_LIBS}
+          DEPENDS ${all_dependencies}
+  )
+endif()
+
 include(onnxruntime_fuzz_test.cmake)

include/onnxruntime/core/common/const_pointer_container.h

@@ -79,6 +79,10 @@ class ConstPointerContainer {
     return data_[index];
   }
 
+  const T* const* data() const {
+    return data_.data();
+  }
+
  private:
   const Container& data_;
 };

include/onnxruntime/core/graph/graph.h

@@ -797,6 +797,9 @@ class Graph {  // NOLINT(clang-analyzer-optin.performance.Padding): preserve exi
   /** Returns true if an initializer value can be overridden by a graph input with the same name. */
   bool CanOverrideInitializer() const noexcept { return ir_version_ >= 4; }
 
+  /** Returns the ONNX IR version for the model. */
+  Version GetOnnxIRVersion() const noexcept { return ir_version_; }
+
   /** returns the initializer's TensorProto if 'name' is an initializer, is constant and
   cannot be overridden at runtime. If the initializer is not found or is not constant, a nullptr is returned.
   @param check_outer_scope If true and the graph is a subgraph,
@@ -812,6 +815,18 @@ class Graph {  // NOLINT(clang-analyzer-optin.performance.Padding): preserve exi
   */
   const ONNX_NAMESPACE::TensorProto* GetInitializer(const std::string& name, bool check_outer_scope) const;
 
+  /// <summary>
+  /// Returns the initializer's TensorProto if 'name' is an initializer (either constant or overridable).
+  /// If the initializer is not found, a nullptr is returned. An output parameter is set to true if the initializer
+  /// is constant.
+  /// </summary>
+  /// <param name="name">The initializer's name.</param>
+  /// <param name="check_outer_scope">Checks outer scope if set to true and the graph is a subgraph.</param>
+  /// <param name="is_constant">Output parameter set to true if the initializer is a constant.</param>
+  /// <returns>The initializer's TensorProto or nullptr.</returns>
+  const ONNX_NAMESPACE::TensorProto* GetInitializer(const std::string& name, bool check_outer_scope,
+                                                    bool& is_constant) const;
+
   /** Gets the Graph inputs excluding initializers.
   These are the required inputs to the Graph as the initializers can be optionally overridden via graph inputs.
   @remarks Contains no nullptr values. */

include/onnxruntime/core/graph/graph_viewer.h

@@ -57,6 +57,11 @@ class GraphViewer {
   /** Returns true if an initializer value can be overridden by a graph input with the same name. */
   bool CanOverrideInitializer() const noexcept;
 
+  /** Returns the ONNX IR version for the model. */
+  Version GetOnnxIRVersion() const noexcept {
+    return graph_->GetOnnxIRVersion();
+  }
+
   /**
   Gets the Graph inputs, excluding initializers.
   @returns Collection of NodeArg pointers for the graph inputs, excluding inputs that have matching initializers.