Get one batch from the data loader of `ds_type`. Optionally `detach` and `denorm`.

def one_batch(self, ds_type:DatasetType=DatasetType.Train, detach:bool=True, denorm:bool=True, cpu:bool=True)->Collection[Tensor]:
        "Get one batch from the data loader of `ds_type`. Optionally `detach` and `denorm`."
        dl = self.dl(ds_type)
        w = self.num_workers
        self.num_workers = 0
        try:     x,y = next(iter(dl))
        finally: self.num_workers = w
        if detach: x,y = to_detach(x,cpu=cpu),to_detach(y,cpu=cpu)
        norm = getattr(self,'norm',False)
        if denorm and norm:
            x = self.denorm(x)
            if norm.keywords.get('do_y',False): y = self.denorm(y, do_x=True)
        return x,y