ConsumerTrackingPipelineVisitor, a PipelineVisitor object.
For internal use only; no backwards-compatibility guarantees.
Visitor for extracting value-consumer relations from the graph.
Tracks the AppliedPTransforms that consume each PValue in the Pipeline. This is used to schedule consuming PTransforms to consume input after the upstream transform has produced and committed output.
Returns a list of side intputs extracted from the graph.
Returns: A list of pvalue.AsSideInput.