Feat autograd mode (#4496)

* feat(AutoGradMode): add AutoGradMode

* style(*): fix typo

* feat(AutoGradMode): refine codes

* style(*): refine codes

* style(*): refine codes

* style(*): use namespace instead of static

* style(AutoGradMode): use class instead of struct

Co-authored-by: oneflow-ci-bot <69100618+oneflow-ci-bot@users.noreply.github.com>

oneflow/core/autograd/autograd_mode.cpp

@@ -0,0 +1,38 @@
+/*
+Copyright 2020 The OneFlow 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 "oneflow/core/autograd/autograd_mode.h"
+
+namespace oneflow {
+
+namespace autograd {
+
+namespace {
+
+bool* GetThreadLocalGradMode() {
+  static thread_local bool g_grad_mode = true;
+  return &g_grad_mode;
+}
+
+}  // namespace
+
+bool GradMode::is_enabled() { return *GetThreadLocalGradMode(); }
+
+void GradMode::set_enabled(bool enabled) { *GetThreadLocalGradMode() = enabled; }
+
+}  // namespace autograd
+
+}  // namespace oneflow

oneflow/core/autograd/autograd_mode.h

@@ -0,0 +1,40 @@
+/*
+Copyright 2020 The OneFlow 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.
+*/
+
+namespace oneflow {
+
+namespace autograd {
+
+struct GradMode {
+  static bool is_enabled();
+  static void set_enabled(bool enabled);
+};
+
+class AutoGradMode {
+ public:
+  AutoGradMode(bool enabled) : prev_mode_(GradMode::is_enabled()) {
+    GradMode::set_enabled(enabled);
+  }
+  ~AutoGradMode() { GradMode::set_enabled(prev_mode_); }
+  bool prev_mode() const { return prev_mode_; }
+
+ private:
+  bool prev_mode_;
+};
+
+}  // namespace autograd
+
+}  // namespace oneflow