The statistics about training and cross validation results over the entire dataset

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In DNN module, each worker outputs the statistics about training and cross validation results such as error rate and the number of instances for partial input data that are assigned to the worker. Even though these statistics show the training progress of neural network models, it will be better to print the statistics over the entire dataset. Aggregation Service can be used for this. In addition, these statistics can be collected using MetricsCollector.