### What changes were proposed in this pull request?
This PR changes the references of the `PartialMerge`/`Final` `AggregateExpression` from `aggBufferAttributes` to `inputAggBufferAttributes`.
After this change, the tests of `SPARK-31620` can fail on the assertion of `QueryTest.assertEmptyMissingInput`. So, this PR also fixes it by overriding the `inputAggBufferAttributes` of the Aggregate operators.
### Why are the changes needed?
With my understanding of Aggregate framework, especially, according to the logic of `AggUtils.planAggXXX`, I think for the `PartialMerge`/`Final` `AggregateExpression` the right references should be `inputAggBufferAttributes`.
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
Before this patch, for an Aggregate operator, its input attributes will always be equal to or more than(because it refers to its own attributes while it should refer to the attributes from the child) its reference attributes. Therefore, its missing inputs must always be empty and break nothing. Thus, it's impossible to add a UT for this patch.
However, after correcting the right references in this PR, the problem is then exposed by `QueryTest.assertEmptyMissingInput` in the UT of SPARK-31620, since missing inputs are no longer always empty. This PR can fix the problem.
Closes#28869 from Ngone51/fix-agg-reference.
Authored-by: yi.wu <yi.wu@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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