Use insert_or_assign instead of erase+emplace (#164868)

insert_or_assign does effectively the same thing as
erase+emplace but more efficiently since the search
does not need to be repeated

Pull Request resolved: https://github.com/pytorch/pytorch/pull/164868
Approved by: https://github.com/eqy

aten/src/ATen/native/cudnn/Conv_v8.cpp

@@ -337,8 +337,7 @@ struct BenchmarkCache {
             engine_cache_order.begin(), engine_cache_order, it->second.second);
       }
     } else {
-      engine_cache.erase(key);
-      engine_cache.emplace(
+      engine_cache.insert_or_assign(
           key,
           std::make_pair(results, engine_cache_order.end())); // dummy iterator
     }

aten/src/ATen/native/cudnn/MHA.cpp

@@ -371,8 +371,7 @@ struct MHAGraphCache {
   }
 
   void update(const KeyType& key, T& results) {
-    engine_cache.erase(key);
-    engine_cache.emplace(key, std::move(results));
+    engine_cache.insert_or_assign(key, std::move(results));
   }
 };