Fix major serialization bug (#503)

Summary:
Pull Request resolved: #503

Found this bug while testing PL on Explat. There is an n^2 loop. The column implementation is already writing the values for each row to the byte buffers (https://fburl.com/code/a48hfogh). This effectively means we are processing this column n^2 times. For 1 million rows that is 1 trillion times we are calling this method 🤦‍♀️

Differential Revision: D43954084

fbshipit-source-id: 0ad2e372eacfa61d4331350814cd0629bd126abe

fbpcf/mpc_std_lib/unified_data_process/serialization/RowStructureDefinition.h

@@ -324,10 +324,8 @@ class RowStructureDefinition : public IRowStructureDefinition<schedulerId> {
       }
     }
 
-    for (int i = 0; i < writeBuffers.size(); i++) {
-      packedBitCol->serializeColumnAsPlaintextBytes(
-          bitPack, writeBuffers, byteOffset);
-    }
+    packedBitCol->serializeColumnAsPlaintextBytes(
+        bitPack, writeBuffers, byteOffset);
   }
 };