Chronos: ProphetForecaster and AutoProphet bugs and enhancements

[TheaperDeng]

• [x] ProphetForecaster should add a param called freq to method predict, since the internal implement defaults the freq to 'D' while it is not always the case.
  • :param freq: freq: the freqency of the predicted dataframe, defaulted to day("D"), the frequency can be anything from the pandas list of frequency strings here: https://pandas.pydata.org/pandas-docs/stable/user_guide/timeseries.html#timeseries-offset-aliases
• [x] AutoProphet should set a hp sample function as the default value for each tunable params.
• [x] AutoProphet takes a dataframe as input, rather than "1-D numpy array" stated in https://analytics-zoo.readthedocs.io/en/latest/doc/PythonAPI/Chronos/autotsestimator.html#chronos-autots-model-auto-prophet
• [x] We should automatically do train/valid split or cross validation (to be discussed) in autoprophet's fit rather than ask user to input a validation dataframe.
• [x] We should add predict, evaluate, save, load in autoprophet's methods.

[TheaperDeng]

@PeterQiu0516 might need your help to confirm if the first three items are bug or just my misusage. I will fix them after your confirmation.

[PeterQiu0516]

• [x] Yes, we could add param freq to the method predict, while Prophet's original predict method does not support param freq (https://github.com/facebook/prophet/blob/master/python/prophet/forecaster.py#L1184) , our customized implementation combine make_future_dataframe and predict together, and we could pass the freq to the internal make_future_dataframe method: https://github.com/intel-analytics/analytics-zoo/blob/master/pyzoo/zoo/chronos/model/prophet.py#L84.
• [x] Yes, and this is already made clear in the comments: https://github.com/intel-analytics/analytics-zoo/blob/master/pyzoo/zoo/chronos/autots/model/auto_prophet.py#L48 I am passing in a value instead of a hp sample function as default only to keep consistent with our other autots API (e.g, auto_lstm: https://github.com/intel-analytics/analytics-zoo/blob/master/pyzoo/zoo/chronos/autots/model/auto_lstm.py#L32)
• [x] Yes, as specified in https://github.com/intel-analytics/analytics-zoo/blob/master/pyzoo/zoo/chronos/model/prophet.py#L57-L59. Thank you for catching up that mistake in comment :)