Add inheritance of ABC from the abc package for abstract classes

[FreeRealEstate221]

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For the abstract classes like forecaster or base_estimator, we could inherit the ABC class from the abc package so that 1) we could prevent abstract classes from being instantiated; 2) we don't need to raise NotImplementedError manually each time

You could find further explanation in the stackoverflow here: https://stackoverflow.com/questions/56008847/when-should-one-inherit-from-abc

[edwinnglabs]

This should reduce the work to implement raise implemented error to all functions. @wangzhishi