[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>

paddle2onnx/mapper/tensor/unstack.cc

@@ -0,0 +1,36 @@
+// Copyright (c) 2022 PaddlePaddle Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "paddle2onnx/mapper/tensor/unstack.h"
+
+namespace paddle2onnx {
+REGISTER_MAPPER(unstack, UnstackMapper)
+
+void UnstackMapper::Opset7() {
+  auto x_info = GetInput("X");
+  auto y_info = GetOutput("Y");
+
+  if (axis_ < 0) {
+    axis_ = axis_ + x_info[0].Rank();
+  }
+
+  auto split_nodes = helper_->Split(
+      x_info[0].name, std::vector<int64_t>(y_info.size(), 1), axis_);
+
+  for (size_t i = 0; i < split_nodes.size(); ++i) {
+    helper_->Squeeze(split_nodes[i], y_info[i].name, {axis_});
+  }
+}
+
+}  // namespace paddle2onnx