scope design doc

doc/refactor/scope.md

+# Scope
+
+### Define
+
+Scope is a context to manage Variables. It mainly contains a map from Variable name to Variable. Net will get and update variable throw scope.
+
+```cpp
+class Scope {
+    Variable GetVar();
+
+private:
+    // var_name -> var
+    std::map<string, Variable> var_map_;
+    Scope* parent_scope_;
+}
+```
+
+You need to specify a scope to run a Net. One net can run in different scopes and update different variable in the scope. If you did not specify one, It will run in a default scope.
+```python
+with ScopeGuard(scope)：
+    Net net = Net();
+    Net.run()
+```
+
+### Chain structure
+
+Scope has a pointer point to it's parent scope, this is mainly used in RNN when it need to create many stepNet.
+
+
+### Scope Guard
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