[SPARK-44340][SQL][FOLLOWUP][3.5] Set partition index correctly for W…

…indowGroupLimitExec

### What changes were proposed in this pull request?
This is a followup of #41899, to set the partition index correctly even if it's not used for now. It also contains a few code cleanup.

### Why are the changes needed?
future-proof

### Does this PR introduce _any_ user-facing change?
'No'.

### How was this patch tested?
existing tests

Closes #42233 from beliefer/SPARK-44340_followup_3.5.

Authored-by: Jiaan Geng <beliefer@163.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

sql/core/src/main/scala/org/apache/spark/sql/execution/window/WindowGroupLimitExec.scala

@@ -72,23 +72,21 @@ case class WindowGroupLimitExec(
     "numOutputRows" -> SQLMetrics.createMetric(sparkContext, "number of output rows"))
 
   protected override def doExecute(): RDD[InternalRow] = {
-    val numOutputRows = longMetric("numOutputRows")
-
     val evaluatorFactory =
       new WindowGroupLimitEvaluatorFactory(
         partitionSpec,
         orderSpec,
         rankLikeFunction,
         limit,
         child.output,
-        numOutputRows)
+        longMetric("numOutputRows"))
 
     if (conf.usePartitionEvaluator) {
       child.execute().mapPartitionsWithEvaluator(evaluatorFactory)
     } else {
-      child.execute().mapPartitionsInternal { iter =>
+      child.execute().mapPartitionsWithIndexInternal { (index, rowIterator) =>
         val evaluator = evaluatorFactory.createEvaluator()
-        evaluator.eval(0, iter)
+        evaluator.eval(index, rowIterator)
       }
     }
   }