[RFC][RUNTIME] Introduce new object protocol. (apache#4115)

* [RUNTIME] Introduce new object protocol.

This PR introduces a new object protocol to unify the node and object.
We also updated the existing runtime::vm code to make use of the new system.

Update to the node will be done in a follow up PR.

Other changes:

- Remove object related code in json serializer as that code logic was not complete
  and we have a separate serializer for VM, can revisit later.

* address review  comment

* Fix the child slot logic

Makefile

@@ -70,7 +70,7 @@ cpplint:
 	python3 3rdparty/dmlc-core/scripts/lint.py vta cpp vta/include vta/src
 	python3 3rdparty/dmlc-core/scripts/lint.py topi cpp topi/include;
 	python3 3rdparty/dmlc-core/scripts/lint.py nnvm cpp nnvm/include nnvm/src;
-	python3 3rdparty/dmlc-core/scripts/lint.py tvm cpp include src verilog\
+	python3 3rdparty/dmlc-core/scripts/lint.py tvm cpp include src \
 	 examples/extension/src examples/graph_executor/src
 
 pylint:

include/tvm/node/node.h

@@ -42,7 +42,7 @@ namespace runtime {
 // forward declaration
 class NDArray;
 // forward declaration
-class Object;
+class ObjectRef;
 }  // namespace runtime
 
 /*!
@@ -63,7 +63,7 @@ class TVM_DLL AttrVisitor {
   virtual void Visit(const char* key, DataType* value) = 0;
   virtual void Visit(const char* key, NodeRef* value) = 0;
   virtual void Visit(const char* key, runtime::NDArray* value) = 0;
-  virtual void Visit(const char* key, runtime::Object* value) = 0;
+  virtual void Visit(const char* key, runtime::ObjectRef* value) = 0;
   template<typename ENum,
            typename = typename std::enable_if<std::is_enum<ENum>::value>::type>
   void Visit(const char* key, ENum* ptr) {

include/tvm/runtime/memory.h

@@ -0,0 +1,115 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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.
+ */
+/*!
+ * \file tvm/runtime/memory.h
+ * \brief Runtime memory management.
+ */
+#ifndef TVM_RUNTIME_MEMORY_H_
+#define TVM_RUNTIME_MEMORY_H_
+
+#include <utility>
+#include <type_traits>
+#include "object.h"
+
+namespace tvm {
+namespace runtime {
+/*!
+ * \brief Allocate an object using default allocator.
+ * \param args arguments to the constructor.
+ * \tparam T the node type.
+ * \return The NodePtr to the allocated object.
+ */
+template<typename T, typename... Args>
+inline ObjectPtr<T> make_object(Args&&... args);
+
+// Detail implementations after this
+//
+// The current design allows swapping the
+// allocator pattern when necessary.
+//
+// Possible future allocator optimizations:
+// - Arena allocator that gives ownership of memory to arena (deleter_= nullptr)
+// - Thread-local object pools: one pool per size and alignment requirement.
+// - Can specialize by type of object to give the specific allocator to each object.
+
+/*!
+ * \brief Base class of object allocators that implements make.
+ *  Use curiously recurring template pattern.
+ *
+ * \tparam Derived The derived class.
+ */
+template<typename Derived>
+class ObjAllocatorBase {
+ public:
+  /*!
+   * \tparam T The type to be allocated.
+   * \tparam Args The constructor signature.
+   * \param args The arguments.
+   */
+  template<typename T, typename... Args>
+  inline ObjectPtr<T> make(Args&&... args) {
+    using Handler = typename Derived::template Handler<T>;
+    static_assert(std::is_base_of<Object, T>::value,
+                  "make_node can only be used to create NodeBase");
+    T* ptr = Handler::New(static_cast<Derived*>(this),
+                         std::forward<Args>(args)...);
+    ptr->type_index_ = T::type_index();
+    ptr->deleter_ = Handler::Deleter();
+    return ObjectPtr<T>(ptr);
+  }
+};
+
+// Simple allocator that uses new/delete.
+class SimpleObjAllocator :
+      public ObjAllocatorBase<SimpleObjAllocator> {
+ public:
+  template<typename T>
+  class Handler {
+   public:
+    template<typename... Args>
+    static T* New(SimpleObjAllocator*, Args&&... args) {
+      // NOTE: the first argument is not needed for SimpleObjAllocator
+      // It is reserved for special allocators that needs to recycle
+      // the object to itself (e.g. in the case of object pool).
+      //
+      // In the case of an object pool, an allocator needs to create
+      // a special chunk memory that hides reference to the allocator
+      // and call allocator's release function in the deleter.
+      return new T(std::forward<Args>(args)...);
+    }
+
+    static Object::FDeleter Deleter() {
+      return Deleter_;
+    }
+
+   private:
+    static void Deleter_(Object* ptr) {
+      delete static_cast<T*>(ptr);
+    }
+  };
+};
+
+template<typename T, typename... Args>
+inline ObjectPtr<T> make_object(Args&&... args) {
+  return SimpleObjAllocator().make<T>(std::forward<Args>(args)...);
+}
+
+}  // namespace runtime
+}  // namespace tvm
+#endif  // TVM_RUNTIME_MEMORY_H_