Improve user method of seeing pipelines generated

Currently, the easiest way for a user to see the pipelines included in the ensemble is through estimator.show_models() which just returns a str which needs to be manually parsed and looked through. There could definitely be a nicer format to view any such pipeline and provide easy access.

[eddiebergman]

From #1206, how to access a specific model in the ensemble

import sklearn
from sklearn import datasets
from autosklearn.classification import AutoSklearnClassifier

X, y = datasets.load_breast_cancer(return_X_y=True)
X_train, X_test, y_train, y_test = sklearn.model_selection.train_test_split(X, y, random_state=1)

clf = AutoSklearnClassifier(time_left_for_this_task=120, per_run_time_limit=30)
clf.fit(X_train, y_train)

# Can find model id's from the `leaderboard()` function
wanted_model_id = ...
wanted_model = None

for (seed, model_id, budget), model in clf.automl_.models_.items():
    if model_id == wanted_model_id:
        wanted_model = model

[mfeurer]

Please open a new issue @TomPham97 for any new questions.