Fix weekly date range bug for deepar

[es94129]

What is changed?

Specify the "W-{weekday}" instead of "W" for the pd.date_range frequency, so that there is no mismatch between the new index and the original time column's dates.

• Previously: new_index_full can start from 2010-02-07 (a Sunday) despite total_min is 2010-02-05 (a Friday), because pd.date_range assumes that a weekly frequency starts from a Sunday (W-SUN)
• Now: the start of new_index_full is aligned with total_min

How is it tested?

• Added unit test: provide a dataframe where the dates are Fridays, and the frequency is "W". Before the fix, the unit test would fail with the target column of the output dataframe(s) being all nan.
• Manual testing: tested with the Walmart Sales dataset, the DeepAR metrics used to be NaN before the change

[sabhya-db]

lgtm