#include <RecoLocalTracker/SiPixelClusterizer/interface/SiPixelClusterProducer.h>
SiPixelClusterProducer is an EDProducer subclass (i.e., a module) which orchestrates clustering of PixelDigis to SiPixelClusters. Consequently, the input is edm::DetSetVector<PixelDigi> and the output is edm::DetSetVector<SiPixelCluster>.
SiPixelClusterProducer invokes one of descendents from PixelClusterizerBase, e.g. PixelThresholdClusterizer (which is the only available option right now). SiPixelClusterProducer loads the PixelDigis, and then iterates over DetIds, invoking PixelClusterizer's clusterizeDetUnit to perform the clustering. clusterizeDetUnit() returns a DetSetVector of SiPixelClusters, which are then recorded in the event.
The calibrations are not loaded at the moment (v1), although that is being planned for the near future.