def helper_test_index_types(self, ig: CovariatesIndexGenerator):
        """test the index type of generated index"""
        # pd.DatetimeIndex
        idx, _ = ig.generate_train_idx(self.target_time, self.cov_time_train)
        assert isinstance(idx, pd.DatetimeIndex)
        idx, _ = ig.generate_inference_idx(
            self.n_short, self.target_time, self.cov_time_inf_short
        )
        assert isinstance(idx, pd.DatetimeIndex)
        idx, _ = ig.generate_train_inference_idx(
            self.n_short, self.target_time, self.cov_time_inf_short
        )
        assert isinstance(idx, pd.DatetimeIndex)
        idx, _ = ig.generate_train_idx(self.target_time, None)
        assert isinstance(idx, pd.DatetimeIndex)

        # pd.RangeIndex
        idx, _ = ig.generate_train_idx(self.target_int, self.cov_int_train)
        assert isinstance(idx, pd.RangeIndex)
        idx, _ = ig.generate_inference_idx(
            self.n_short, self.target_int, self.cov_int_inf_short
        )
        assert isinstance(idx, pd.RangeIndex)
        idx, _ = ig.generate_train_inference_idx(
            self.n_short, self.target_int, self.cov_int_inf_short
        )
        assert isinstance(idx, pd.RangeIndex)
        idx, _ = ig.generate_train_idx(self.target_int, None)
        assert isinstance(idx, pd.RangeIndex)
test the index type of generated index
darts/tests/dataprocessing/encoders/test_covariate_index_generators.py