## What changes were proposed in this pull request?
In https://github.com/apache/spark/pull/18064, we allowed `RunnableCommand` to have children in order to fix some UI issues. Then we made `InsertIntoXXX` commands take the input `query` as a child, when we do the actual writing, we just pass the physical plan to the writer(`FileFormatWriter.write`).
However this is problematic. In Spark SQL, optimizer and planner are allowed to change the schema names a little bit. e.g. `ColumnPruning` rule will remove no-op `Project`s, like `Project("A", Scan("a"))`, and thus change the output schema from "<A: int>" to `<a: int>`. When it comes to writing, especially for self-description data format like parquet, we may write the wrong schema to the file and cause null values at the read path.
Fortunately, in https://github.com/apache/spark/pull/18450 , we decided to allow nested execution and one query can map to multiple executions in the UI. This releases the major restriction in #18604 , and now we don't have to take the input `query` as child of `InsertIntoXXX` commands.
So the fix is simple, this PR partially revert #18064 and make `InsertIntoXXX` commands leaf nodes again.
## How was this patch tested?
new regression test
Author: Wenchen Fan <wenchen@databricks.com>
Closes#19474 from cloud-fan/bug.
274f0efefa