AttributeError: 'Pipeline' object has no attribute 'pred_dist'

Hey!

Let me begin by thanking y'all for a fantastic package!! I was wanting to use tpot for finding a good sklearn pipeline with [ngboost](https://github.com/stanfordmlgroup/ngboost) as the final estimator, for a regression problem with one output variable. However, since the probabilistic prediction method for ngboost is called 'pred_dist', which is not something sklearn supports, python throws an error. Do you have any suggestions?