Public API / documentation for # generations executed

The number of generations _executed_ (not _planned_ as in the `generations` param) is prominently featured in `TPOTEstimator's` output, but doesn't appear to be accessible via its public API. I am testing progressively longer hyperparameter optimization of several ML models using TPOT2, and it would be useful to get this information directly from public / documented API that I have some assurance will be durable over time.

The best / only way I've found to get this so far is from `estimator._evolver_instance.generation`. `estimator.evaluated_individuals` seems to have the first generation each model was introduced, which isn't informative in recording when / why my optimization hit an early stop limit.