text: Title: TimeSeriesForecaster: Predict function returns ValueError
  Body: Hi,
  
  I've been using AutoKeras for Time Series forecasting but when the model has been trained and I apply the test data, it raises: "ValueError: The prediction data requires the original training data to make predictions on subsequent data points"
  but I can't put both the test data and the training data as separate arguments? Do I need to append the test data to the training data for the model to predict or is there something I'm missing?
  
  
   Setup Details
   - OS type and version: Windows 10
   - Python: 3.9
   - autokeras: 1.0.16
   - keras-tuner: 1.0.4
   - scikit-learn: 1.0
   - numpy: 1.19.5
   - pandas: 1.3.4
   - tensorflow: 2.5.0 (GPU)
  
  