removing start_date and end_date from configs

[joker2411]

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[dpatchigolla]

Let's skip this. If end_date is none, we go to pythonWHT.get_date_range, irrespective of start_date.
We always check only for end_date. If it's None, we go to pythonWHT.
If it's not None, we then check for start_date, only for sanity. If (end_date - start_date ) is less than 2 times prediction_horizon_days, we raise an exception saying begin_time and end_time need to be atleast 2*prediction_horizon_days apart for the predictive feature.

If (end_date - start_date ) is greather than or equal to 2t times prediction_horizon_days, we again make a call to get_date_range with end_date as parameter (creation_ts=end_date).