text: Title: AttributeError: 'Merge' object has no attribute 'fit_transform'
  Body: Hi everyone,
  
  I got a problem when running the example from autokeras website. It returns "AttributeError: 'Merge' object has no attribute 'fit_transform'". It reports the same error when I do not use Merge(), say only use ConvBlock (output_node1). Any help would be appreciated.
  
  Here is the code:
  import autokeras as ak
  from tensorflow.keras.datasets import mnist
  input_node = ak.ImageInput()
  output_node = ak.Normalization()(input_node)
  output_node = ak.ImageAugmentation()(output_node)
  output_node1 = ak.ConvBlock()(output_node)
  output_node2 = ak.ResNetBlock(version='v2')(output_node)
  output_node = ak.Merge()([output_node1, output_node2])
  
  auto_model = ak.AutoModel(
      inputs=input_node, 
      outputs=output_node,
      max_trials=10)
  
  (x_train, y_train), (x_test, y_test) = mnist.load_data()
  print(x_train.shape) # (60000, 28, 28)
  print(y_train.shape) # (60000,)
  print(y_train[:3]) # array([7, 2, 1], dtype=uint8)
  
  \# Feed the AutoModel with training data.
  auto_model.fit(x_train, y_train)
  \# Predict with the best model.
  predicted_y = auto_model.predict(x_test)
  \# Evaluate the best model with testing data.
  print(auto_model.evaluate(x_test, y_test))
  
  The version information and I use GPU:
  python --version  Python 3.6.9 .
  tf.__version__ '2.1.0'
  tf.keras.__version__ '2.2.4-tf'
  autokeras-1.0.0.
  
  
  # Here are all error messages:
  AttributeError                            Traceback (most recent call last)
  <ipython-input-9-0fc492058b0a> in <module>
       21 
       22 # Feed the AutoModel with training data.
  ---> 23 auto_model.fit(x_train, y_train)
       24 # Predict with the best model.
       25 predicted_y = auto_model.predict(x_test)
  
  ~/software/miniconda3/envs/gpu/lib/python3.6/site-packages/autokeras/auto_model.py in fit(self, x, y, epochs, callbacks, validation_split, validation_data, **kwargs)
      185             y=y,
      186             validation_data=validation_data,
  --> 187             validation_split=validation_split)
      188 
      189         # Initialize the hyper_graph.
  
  ~/software/miniconda3/envs/gpu/lib/python3.6/site-packages/autokeras/auto_model.py in _prepare_data(self, x, y, validation_data, validation_split)
      260         # TODO: Handle other types of input, zip dataset, tensor, dict.
      261         # Prepare the dataset.
  --> 262         dataset = self._process_xy(x, y, fit=True)
      263         if validation_data:
      264             self._split_dataset = False
  
  ~/software/miniconda3/envs/gpu/lib/python3.6/site-packages/autokeras/auto_model.py in _process_xy(self, x, y, fit, predict)
      244             for data, head_block in zip(y, self.heads):
      245                 if fit:
  --> 246                     data = head_block.fit_transform(data)
      247                 else:
      248                     data = head_block.transform(data)
  
  AttributeError: 'Merge' object has no attribute 'fit_transform'