Call `train_tfm` and `valid_tfm` after opening image, before converting from `PIL.Image`

def _db_pre_transform(self, train_tfm:List[Callable], valid_tfm:List[Callable]):
    "Call `train_tfm` and `valid_tfm` after opening image, before converting from `PIL.Image`"
    self.train_ds.x.after_open = compose(train_tfm)
    self.valid_ds.x.after_open = compose(valid_tfm)
    return self