Public API / documentation for # generations executed

[chimaerase]

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.