def walk(self, win, step, start, end, join_channels):
        raise NotImplementedError
Returns tuple (install counts dict, datetime) from Datastore.
dataset/ETHPy150Open dongying/dear/dear/spectrum/_base.py/SpectrumBase.walk--crossover with--dataset/ETHPy150Open google/simian/src/simian/mac/models/base.py/ReportsCache.GetInstallCounts