`CoalesceBatches` physical optimizer rule says it should be last but it isn't

[Jefffrey]

datafusion/datafusion/physical-optimizer/src/optimizer.rs

Lines 123 to 129 in e323357

	// The CoalesceBatches rule will not influence the distribution and ordering of the
	// whole plan tree. Therefore, to avoid influencing other rules, it should run last.
	Arc::new(CoalesceBatches::new()),
	Arc::new(CoalesceAsyncExecInput::new()),
	// Remove the ancillary output requirement operator since we are done with the planning
	// phase.
	Arc::new(OutputRequirements::new_remove_mode()),

Can see here it states CoalesceBatches rule should be last but seems this isn't the case anymore; either comment is outdated and it doesn't need to be last anymore, or we should in-fact move it down to respect the comment.

[Jefffrey]

Side note: it would be nice to add the comment for CoalesceAsyncExecInput too, to be consistent with how it's done for other rules. Something like:

// Coalesce in order to reduce number of async function invocations for efficiency.

[alamb]

FWIW I am working to organize some work to remove this execution plan entirely:

• [EPIC] Remove CoalesceBatchesExec operator #18779