public class GroupByConstParallelSetter
Rule: If a LOCogroup is 'group all', set the parallelism to 1,
or in general - if the group-by expression is just a constant
then set parallelism to 1
LogicalExpressionSimplifier could be used to convert an expression
with constants into a single ConstantExpression
Get the transformer for this rule. Abstract because the rule
may want to choose how to instantiate the transformer.
This should never return a cached transformer, it should
always return a fresh one with no state.