Minor: include output partition count of RepartitionExec to tree explain (apache#15717)

* add output partition count to RepartitionExec's tree explain

* Review: better display

Author: 2010YOUY01

datafusion/physical-plan/src/repartition/mod.rs

@@ -508,11 +508,18 @@ impl DisplayAs for RepartitionExec {
             }
             DisplayFormatType::TreeRender => {
                 writeln!(f, "partitioning_scheme={}", self.partitioning(),)?;
+
+                let input_partition_count =
+                    self.input.output_partitioning().partition_count();
+                let output_partition_count = self.partitioning().partition_count();
+                let input_to_output_partition_str =
+                    format!("{} -> {}", input_partition_count, output_partition_count);
                 writeln!(
                     f,
-                    "input_partition_count={}",
-                    self.input.output_partitioning().partition_count()
+                    "partition_count(in->out)={}",
+                    input_to_output_partition_str
                 )?;
+
                 if self.preserve_order {
                     writeln!(f, "preserve_order={}", self.preserve_order)?;
                 }