add classes_ to PytorchLRClassifier

[perib]

[please review the Contribution Guidelines prior to submitting your pull request. go ahead and delete this line if you've already reviewed said guidelines.]

What does this PR do?

add a classes_ attribute to the pytorch classifiers

Where should the reviewer start?

How should this PR be tested?

from tpot import TPOTClassifier
from sklearn.datasets import make_blobs
from sklearn.model_selection import train_test_split

X, y = make_blobs(n_samples=100, centers=2, n_features=3, random_state=42)
X_train, X_test, y_train, y_test = train_test_split(X, y, train_size=0.75, test_size=0.25)

clf = TPOTClassifier(config_dict='TPOT NN', template='Selector-Transformer-PytorchLRClassifier',
verbosity=2, population_size=2, generations=2)
clf.fit(X_train, y_train)
print(clf.score(X_test, y_test))
clf.export('tpot_nn_demo_pipeline.py')

What are the relevant issues?

#1339