Calculate min/max and histogram inside the CustomAggregator op
Calculate min/max and histogram inside the CustomAggregator op

This cl is the first part of removing the Calibration singleton:
- Part 1 (this cl): Move the min/max and histogram calculation inside CustomAggregator op.
- Part 2 (follow-up cl): Aggregate the statistics inside StatisticsSaver op and remove the singleton.
