Merge branch 'MS-351' into 'branch-0.4.0'

fix bug unittest

Closes MS-351

See merge request megasearch/milvus!360

Former-commit-id: 517c0988845566d2d26a51930e674d7407b38be9

cpp/unittest/server/cache_test.cpp

@@ -58,6 +58,14 @@ public:
 
     }
 
+    engine::VecIndexPtr CopyToGpu(const int64_t &device_id, const engine::Config &cfg) override {
+
+    }
+
+    engine::VecIndexPtr CopyToCpu(const engine::Config &cfg) override {
+
+    }
+
     virtual int64_t Dimension() {
         return dimension_;
     }