chore: import upstream snapshot with attribution

paddle/utils/string/to_string.h

@@ -0,0 +1,81 @@
+/* Copyright (c) 2016 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. */
+
+#pragma once
+#include <sstream>
+#include <string>
+#include <type_traits>
+#include <typeindex>
+#include <vector>
+
+namespace paddle {
+namespace string {
+inline std::ostream& operator<<(std::ostream& s, const std::type_index& t) {
+  s << t.name();
+  return s;
+}
+
+template <typename T,
+          typename std::enable_if<!std::is_enum<T>::value, int>::type = 0>
+inline std::string to_string(T v) {
+  std::ostringstream sout;
+  sout << v;
+  return sout.str();
+}
+
+template <typename T,
+          typename std::enable_if<std::is_enum<T>::value, int>::type = 0>
+inline std::string to_string(T v) {
+  return std::to_string(static_cast<int>(v));
+}
+
+template <>
+inline std::string to_string(std::type_index t) {
+  return t.name();
+}
+
+// Faster std::string/const char* type
+template <>
+inline std::string to_string(std::string v) {
+  return v;
+}
+
+template <>
+inline std::string to_string(const char* v) {
+  return std::string(v);
+}
+
+inline std::ostream& operator<<(std::ostream& os,
+                                const std::vector<std::vector<size_t>>& lod) {
+  os << "{";
+  for (auto& v : lod) {
+    os << "{";
+    bool is_first = true;
+    for (auto& i : v) {
+      if (is_first) {
+        os << i;
+        is_first = false;
+      } else {
+        os << ", " << i;
+      }
+    }
+    os << "}";
+  }
+  os << "}";
+
+  return os;
+}
+
+}  // namespace string
+}  // namespace paddle