[Mobile][ARM]fix possible memory leak of tensor in CPUContext.test=mobile (#2695)

mobile/src/framework/context.h

@@ -44,15 +44,13 @@ namespace framework {
 struct CPUContext {
  private:
   CPUContext();
-  virtual ~CPUContext() {}
 
  public:
+  ~CPUContext() {}
+
   static CPUContext* Context() {
-    static CPUContext* ctx = nullptr;
-    if (ctx == nullptr) {
-      ctx = new CPUContext();
-    }
-    return ctx;
+    static CPUContext ctx;
+    return &ctx;
   }
 
   void set_thread_num(int thread_num,