[SPARK-15438][SQL] improve explain of whole stage codegen

## What changes were proposed in this pull request?

Currently, the explain of a query with whole-stage codegen looks like this
```
>>> df = sqlCtx.range(1000);df2 = sqlCtx.range(1000);df.join(pyspark.sql.functions.broadcast(df2), 'id').explain()
== Physical Plan ==
WholeStageCodegen
:  +- Project [id#1L]
:     +- BroadcastHashJoin [id#1L], [id#4L], Inner, BuildRight, None
:        :- Range 0, 1, 4, 1000, [id#1L]
:        +- INPUT
+- BroadcastExchange HashedRelationBroadcastMode(List(input[0, bigint]))
   +- WholeStageCodegen
      :  +- Range 0, 1, 4, 1000, [id#4L]
```

The problem is that the plan looks much different than logical plan, make us hard to understand the plan (especially when the logical plan is not showed together).

This PR will change it to:

```
>>> df = sqlCtx.range(1000);df2 = sqlCtx.range(1000);df.join(pyspark.sql.functions.broadcast(df2), 'id').explain()
== Physical Plan ==
*Project [id#0L]
+- *BroadcastHashJoin [id#0L], [id#3L], Inner, BuildRight, None
   :- *Range 0, 1, 4, 1000, [id#0L]
   +- BroadcastExchange HashedRelationBroadcastMode(List(input[0, bigint, false]))
      +- *Range 0, 1, 4, 1000, [id#3L]
```

The `*`before the plan means that it's part of whole-stage codegen, it's easy to understand.

## How was this patch tested?

Manually ran some queries and check the explain.

Author: Davies Liu <davies@databricks.com>

Closes #13204 from davies/explain_codegen.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala

@@ -467,50 +467,7 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
   }
 
   /**
-   * All the nodes that will be used to generate tree string.
-   *
-   * For example:
-   *
-   *   WholeStageCodegen
-   *   +-- SortMergeJoin
-   *       |-- InputAdapter
-   *       |   +-- Sort
-   *       +-- InputAdapter
-   *           +-- Sort
-   *
-   * the treeChildren of WholeStageCodegen will be Seq(Sort, Sort), it will generate a tree string
-   * like this:
-   *
-   *   WholeStageCodegen
-   *   : +- SortMergeJoin
-   *   :    :- INPUT
-   *   :    :- INPUT
-   *   :-  Sort
-   *   :-  Sort
-   */
-  protected def treeChildren: Seq[BaseType] = children
-
-  /**
-   * All the nodes that are parts of this node.
-   *
-   * For example:
-   *
-   *   WholeStageCodegen
-   *   +- SortMergeJoin
-   *      |-- InputAdapter
-   *      |   +-- Sort
-   *      +-- InputAdapter
-   *          +-- Sort
-   *
-   * the innerChildren of WholeStageCodegen will be Seq(SortMergeJoin), it will generate a tree
-   * string like this:
-   *
-   *   WholeStageCodegen
-   *   : +- SortMergeJoin
-   *   :    :- INPUT
-   *   :    :- INPUT
-   *   :-  Sort
-   *   :-  Sort
+   * All the nodes that are parts of this node, this is used by subquries.
    */
   protected def innerChildren: Seq[BaseType] = Nil
 
@@ -522,7 +479,10 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
    * `lastChildren` for the root node should be empty.
    */
   def generateTreeString(
-      depth: Int, lastChildren: Seq[Boolean], builder: StringBuilder): StringBuilder = {
+      depth: Int,
+      lastChildren: Seq[Boolean],
+      builder: StringBuilder,
+      prefix: String = ""): StringBuilder = {
     if (depth > 0) {
       lastChildren.init.foreach { isLast =>
         val prefixFragment = if (isLast) "   " else ":  "
@@ -533,6 +493,7 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
       builder.append(branch)
     }
 
+    builder.append(prefix)
     builder.append(simpleString)
     builder.append("\n")
 
@@ -542,9 +503,9 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
       innerChildren.last.generateTreeString(depth + 2, lastChildren :+ false :+ true, builder)
     }
 
-    if (treeChildren.nonEmpty) {
-      treeChildren.init.foreach(_.generateTreeString(depth + 1, lastChildren :+ false, builder))
-      treeChildren.last.generateTreeString(depth + 1, lastChildren :+ true, builder)
+    if (children.nonEmpty) {
+      children.init.foreach(_.generateTreeString(depth + 1, lastChildren :+ false, builder, prefix))
+      children.last.generateTreeString(depth + 1, lastChildren :+ true, builder, prefix)
     }
 
     builder

sql/core/src/main/scala/org/apache/spark/sql/execution/WholeStageCodegenExec.scala

@@ -245,9 +245,13 @@ case class InputAdapter(child: SparkPlan) extends UnaryExecNode with CodegenSupp
      """.stripMargin
   }
 
-  override def simpleString: String = "INPUT"
-
-  override def treeChildren: Seq[SparkPlan] = Nil
+  override def generateTreeString(
+      depth: Int,
+      lastChildren: Seq[Boolean],
+      builder: StringBuilder,
+      prefix: String = ""): StringBuilder = {
+    child.generateTreeString(depth, lastChildren, builder, "")
+  }
 }
 
 object WholeStageCodegenExec {
@@ -398,20 +402,13 @@ case class WholeStageCodegenExec(child: SparkPlan) extends UnaryExecNode with Co
      """.stripMargin.trim
   }
 
-  override def innerChildren: Seq[SparkPlan] = {
-    child :: Nil
-  }
-
-  private def collectInputs(plan: SparkPlan): Seq[SparkPlan] = plan match {
-    case InputAdapter(c) => c :: Nil
-    case other => other.children.flatMap(collectInputs)
+  override def generateTreeString(
+      depth: Int,
+      lastChildren: Seq[Boolean],
+      builder: StringBuilder,
+      prefix: String = ""): StringBuilder = {
+    child.generateTreeString(depth, lastChildren, builder, "*")
   }
-
-  override def treeChildren: Seq[SparkPlan] = {
-    collectInputs(child)
-  }
-
-  override def simpleString: String = "WholeStageCodegen"
 }
 
 

sql/core/src/main/scala/org/apache/spark/sql/execution/exchange/Exchange.scala

@@ -60,9 +60,6 @@ case class ReusedExchangeExec(override val output: Seq[Attribute], child: Exchan
   override protected[sql] def doExecuteBroadcast[T](): broadcast.Broadcast[T] = {
     child.executeBroadcast()
   }
-
-  // Do not repeat the same tree in explain.
-  override def treeChildren: Seq[SparkPlan] = Nil
 }
 
 /**