[Build] Support build with source code of Paddle2ONNX (#1559)

* Add notes for tensors

* Optimize some apis

* move some warnings

* Support build with Paddle2ONNX

* Add protobuf support

* Fix compile on mac

* add clearn package script

* Add paddle2onnx code

* remove submodule

* Add onnx ocde

* remove softlink

* add onnx code

* fix error

* Add cmake file

* fix patchelf

* update paddle2onnx

* Delete .gitmodules

---------

Co-authored-by: PaddleCI <paddle_ci@example.com>
Co-authored-by: pangyoki <pangyoki@126.com>
Co-authored-by: jiangjiajun <jiangjiajun@baidu.lcom>

third_party/optimizer/examples/onnx_optimizer_exec.cpp

@@ -0,0 +1,31 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <onnxoptimizer/optimize.h>
+
+#include <onnx/checker.h>
+#include <onnx/onnx_pb.h>
+
+#include <fstream>
+
+int main(int argc, char **argv) {
+  ONNX_NAMESPACE::ModelProto model;
+  std::ifstream ifs(argv[1]);
+  bool success = model.ParseFromIstream(&ifs);
+  if (!success) {
+    std::cout << "load failed" << std::endl;
+    return -1;
+  }
+  onnx::checker::check_model(model);
+  const auto new_model = onnx::optimization::Optimize(
+      model, onnx::optimization::GetFuseAndEliminationPass());
+  onnx::checker::check_model(new_model);
+  std::ofstream ofs(argv[2]);
+  success = new_model.SerializePartialToOstream(&ofs);
+  if (!success) {
+    std::cout << "save failed" << std::endl;
+    return -1;
+  }
+  return 0;
+}