Frozen models use the same tuning parameters as their parent model
instead of independently optimizing them to allow efficiently
retraining models on larger amounts of the training data.
In addition to trainingRowCount and trainingDuration, frozen datetime models
may be trained on an exact date range. Only one of trainingRowCount,
trainingDuration, or trainingStartDate and trainingEndDate should be specified.
Models specified using trainingStartDate and trainingEndDate are the only ones
that can be trained into the holdout data (once the holdout is unlocked).