Helper function to concat two NDArrays.

def _concat(self, first_data, second_data):
        """Helper function to concat two NDArrays."""
        assert len(first_data) == len(
            second_data), 'data source should contain the same size'
        if first_data and second_data:
            return [
                concat(
                    first_data[x],
                    second_data[x],
                    dim=0
                ) for x in range(len(first_data))
            ]
        elif (not first_data) and (not second_data):
            return []
        else:
            return [
                first_data[0] if first_data else second_data[0]
                for x in range(len(first_data))
            ]