Process the inner datasets.

def process(self):
        "Process the inner datasets."
        xp,yp = self.get_processors()
        for ds,n in zip(self.lists, ['train','valid','test']): ds.process(xp, yp, name=n)
        #progress_bar clear the outputs so in some case warnings issued during processing disappear.
        for ds in self.lists:
            if getattr(ds, 'warn', False): warn(ds.warn)
        return self