text: Title: AutoKeras with tensorflow.dataset gives TypeError: 'PrefetchDataset' object does not support indexing
  Body: 
  ### Bug Description
  ---------------------------------------------------------------------------
  TypeError                                 Traceback (most recent call last)
  <ipython-input-18-e8d1a7f824a0> in <module>
        1 # Feed the image classifier with training data.
  ----> 2 clf.fit(train_ds[0], train_ds[1])
  
  TypeError: 'PrefetchDataset' object does not support indexing
  
  ### Reproducing Steps
  Following the steps for creating a tf.dataset from: https://www.tensorflow.org/tutorials/load_data/images
  
  I encountered an issue with AutoModel when calling:
  clf.fit(train_ds[0], train_ds[1])
  and
  clf.fit(next(iter(train_ds))[0], next(iter(train_ds))[1])
  gives : 
  TypeError: Expect the data to ImageInput to be numpy.ndarray or tf.data.Dataset, but got <class 'tensorflow.python.framework.ops.EagerTensor'>.
  
  ### Expected Behavior
  I expect to be able to use AutoKeras with imagesets that are too large to fit in memory. 
  
  
  