Minor: Add comment on input_schema from AggregateExec (#7727)

* Remove input_schema

* Revert "Remove input_schema"

This reverts commit 5096883.

* Add comment

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

@@ -285,7 +285,9 @@ pub struct AggregateExec {
     schema: SchemaRef,
     /// Input schema before any aggregation is applied. For partial aggregate this will be the
     /// same as input.schema() but for the final aggregate it will be the same as the input
-    /// to the partial aggregate
+    /// to the partial aggregate, i.e., partial and final aggregates have same `input_schema`.
+    /// We need the input schema of partial aggregate to be able to deserialize aggregate
+    /// expressions from protobuf for final aggregate.
     pub input_schema: SchemaRef,
     /// The columns map used to normalize out expressions like Partitioning and PhysicalSortExpr
     /// The key is the column from the input schema and the values are the columns from the output schema