<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def transform_with(self, estimator, out_ds, fmt=None):
 """Call the partial_transform method of the estimator on this dataset Parameters estimator : object with ``partial_fit`` method This object will be used to transform this dataset into a new dataset. The estimator should be fitted prior to calling this method. out_ds : str or Dataset This dataset will be transformed and saved into out_ds. If out_ds is a path, a new dataset will be created at that path. fmt : str The type of dataset to create if out_ds is a string. Returns ------- out_ds : Dataset The tranformed dataset. """

        if isinstance(out_ds, str):
            out_ds = self.create_derived(out_ds, fmt=fmt)
        elif isinstance(out_ds, _BaseDataset):
            err = "Dataset must be opened in write mode."
            assert out_ds.mode in ('w', 'a'), err
        else:
            err = "Please specify a dataset path or an existing dataset."
            raise ValueError(err)
        for key in self.keys():
            out_ds[key] = estimator.partial_transform(self[key])
        return out_ds