[SPARK-18189][SQL] Fix serialization issue in KeyValueGroupedDataset

## What changes were proposed in this pull request?
Likewise [DataSet.scala](https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala#L156) KeyValueGroupedDataset should mark the queryExecution as transient.

As mentioned in the Jira ticket, without transient we saw serialization issues like

```
Caused by: java.io.NotSerializableException: org.apache.spark.sql.execution.QueryExecution
Serialization stack:
        - object not serializable (class: org.apache.spark.sql.execution.QueryExecution, value: ==
```

## How was this patch tested?

Run the query which is specified in the Jira ticket before and after:
```
val a = spark.createDataFrame(sc.parallelize(Seq((1,2),(3,4)))).as[(Int,Int)]
val grouped = a.groupByKey(
{x:(Int,Int)=>x._1}
)
val mappedGroups = grouped.mapGroups((k,x)=>
{(k,1)}
)
val yyy = sc.broadcast(1)
val last = mappedGroups.rdd.map(xx=>
{ val simpley = yyy.value 1 }
)
```

Author: Ergin Seyfe <eseyfe@fb.com>

Closes #15706 from seyfe/keyvaluegrouped_serialization.

1 files changed, 1 insertions, 1 deletions

diff --git a/sql/core/src/main/scala/org/apache/spark/sql/KeyValueGroupedDataset.scala b/sql/core/src/main/scala/org/apache/spark/sql/KeyValueGroupedDataset.scala
index 4cb0313aa9..31ce8eb25e 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/KeyValueGroupedDataset.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/KeyValueGroupedDataset.scala
@@ -40,7 +40,7 @@ import org.apache.spark.sql.expressions.ReduceAggregator
 class KeyValueGroupedDataset[K, V] private[sql](
     kEncoder: Encoder[K],
     vEncoder: Encoder[V],
-    val queryExecution: QueryExecution,
+    @transient val queryExecution: QueryExecution,
     private val dataAttributes: Seq[Attribute],
     private val groupingAttributes: Seq[Attribute]) extends Serializable {