[fix](nereids) check failed that exchange node under agg must from PhysicalDistribute (#15473)

when nereids translates PhysicalHashAggreg node to original plan, if the input fragment root is exchange node, nereids assumes that this exchanged node is generated from PhyscialDistirbute node.
But this assumption is not true. For example, sort node could be translated to exchange(merge phase)+sort(local phase).

fe/fe-core/src/main/java/org/apache/doris/nereids/glue/translator/PhysicalPlanTranslator.java

@@ -227,10 +227,14 @@ public class PhysicalPlanTranslator extends DefaultPlanVisitor<PlanFragment, Pla
                 .collect(Collectors.toCollection(ArrayList::new));
 
         PlanFragment currentFragment;
-        if (inputPlanFragment.getPlanRoot() instanceof ExchangeNode) {
-            Preconditions.checkState(aggregate.child() instanceof PhysicalDistribute,
-                    "When the ExchangeNode is child of PhysicalHashAggregate, "
-                            + "it should be created by PhysicalDistribute, but meet " + aggregate.child());
+        if (inputPlanFragment.getPlanRoot() instanceof ExchangeNode
+                && aggregate.child() instanceof PhysicalDistribute) {
+            //the exchange node is generated in two cases:
+            //  1. some nodes (e.g. sort node) need to gather data from multiple instances, and hence their gather phase
+            //     need an exchange node. For this type of exchange, their data partition is un_partitioned, do not
+            //     create a new plan fragment.
+            //  2. PhysicalDistribute node is translated to exchange node. PhysicalDistribute node means we need to
+            //     shuffle data, and we have to create a new plan fragment.
             ExchangeNode exchangeNode = (ExchangeNode) inputPlanFragment.getPlanRoot();
             Optional<List<Expression>> partitionExpressions = aggregate.getPartitionExpressions();
             PhysicalDistribute physicalDistribute = (PhysicalDistribute) aggregate.child();