feat(imperative/python): add transformation manager

GitOrigin-RevId: a3c1732ffd4fafdde27c69fcdc3488ae19ec3dd0

imperative/python/src/transformation.h

+/**
+ * \file imperative/python/src/transformation.h
+ * MegEngine is Licensed under the Apache License, Version 2.0 (the "License")
+ *
+ * Copyright (c) 2014-2021 Megvii Inc. All rights reserved.
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT ARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ */
+
+#pragma once
+
+#include "megbrain/imperative/transformation.h"
+
+namespace mgb::imperative::python {
+struct TransformationManager {
+    enum Segment {
+        ModuleTrace,
+        Grad,
+        Scalar,
+        Trace,
+        Eval,
+    };
+
+    std::array<std::vector<std::shared_ptr<Transformation>>, 5> segments;
+
+    template <Segment segment>
+    void register_at(std::shared_ptr<Transformation> transformation) {
+        mgb_assert(segment < segments.size());
+        std::shared_ptr<Transformation> next;
+        for (size_t i = segment; i < segments.size(); ++i) {
+            if (!segments[i].empty()) {
+                next = segments[i].back();
+                break;
+            }
+        }
+        if (!next) {
+            transformation->register_at(Transformation::bottom());
+        } else {
+            transformation->register_at(next->pos());
+        }
+        segments[segment].push_back(transformation);
+    }
+
+    template <Segment segment>
+    void unregister(std::shared_ptr<Transformation> transformation) noexcept {
+        mgb_assert(segment < segments.size());
+        auto iter = std::find(
+                segments[segment].begin(), segments[segment].end(), transformation);
+        mgb_assert(iter != segments[segment].end());
+        transformation->unregister();
+        segments[segment].erase(iter);
+    }
+
+    static TransformationManager& get_instance() {
+        static TransformationManager sl_instance;
+        return sl_instance;
+    }
+};
+}  // namespace mgb::imperative::python