Switch default `replicate_model_fn.loss_reduction` to SUM_BY_NONZERO_WEIGHTS.

Users of weighted losses need loss_reduction=SUM, otherwise the most common choice is SUM_BY_NONZERO_WEIGHTS.  Estimator class doesn't have visibility into whether weighted losses are used, so the default was assumed to be SUM, because it's correct-ish in either case.  This change sets the default to SUM_BY_NONZERO_WEIGHTS to accomodate the most common case.

PiperOrigin-RevId: 182122737