text: Title: ImageClassifier no longer has final_fit (AttributeError: 'ImageClassifier' object has no attribute 'final_fit')
  Body: 
  ### Bug Description
  ---------------------------------------------------------------------------
  AttributeError: 'ImageClassifier' object has no attribute 'final_fit'
  ---------------------------------------------------------------------------
  
  I see examples such as: 
  clf = ImageClassifier(verbose=True)
  clf.fit(x_train, y_train, time_limit=12 * 60 * 60)
  clf.final_fit(x_train, y_train, x_test, y_test, retrain=True)
  y = clf.evaluate(x_test, y_test)
  print(y)
  But when I run something similar I get the error: 
  AttributeError                            Traceback (most recent call last)
  <ipython-input-8-4e011cd6ccac> in <module>
  ----> 1 clf.final_fit(trainX, trainY, testX, testY, retrain=True)
        2 keras_model = clf.export_model()
  
  AttributeError: 'ImageClassifier' object has no attribute 'final_fit'
  
  
  As suggested here: https://github.com/keras-team/autokeras/issues/186 
  I have tried adding the activation function and retraining on the data... but to do so I still end up choosing many hyper parameters such as LR and optimizer...
  I thought that was sorta the point of Auto ML?
  
  What am I missing? 
  