Add normalize transform using `stats` (defaults to `DataBunch.batch_stats`)

def normalize(self, stats:Collection[Tensor]=None, do_x:bool=True, do_y:bool=False)->None:
        "Add normalize transform using `stats` (defaults to `DataBunch.batch_stats`)"
        if getattr(self,'norm',False): raise Exception('Can not call normalize twice')
        if stats is None: self.stats = self.batch_stats()
        else:             self.stats = stats
        self.norm,self.denorm = normalize_funcs(*self.stats, do_x=do_x, do_y=do_y)
        self.add_tfm(self.norm)
        return self