Decouple updating t from update_stats

[jackgerrits]

The t value used to decay learning rate among other things is updated during update_stats. It would be better if t was updated automatically during learn instead of in a "finalization" method. That way update_stats could he be avoided entirely if loss is not needed.

We would need to update the semantics of t to only update during learn and then change this call to remove holdout and unlabeled references.

vowpal_wabbit/vowpalwabbit/core/src/reductions/gd.cc

Line 772 in a9b305f

float t = static_cast<float>(

Confirmation on if updating t during only learn makes sense, but it seems to me to be valid.

We should also consider moving t outside of shared_data since all other fields are reporting related only.