Return Series with number of distinct
           observations over requested axis.

        Args:
            axis : {0 or 'index', 1 or 'columns'}, default 0
            dropna : boolean, default True

        Returns:
            nunique : Series

def nunique(self, axis=0, dropna=True):
        """Return Series with number of distinct
           observations over requested axis.

        Args:
            axis : {0 or 'index', 1 or 'columns'}, default 0
            dropna : boolean, default True

        Returns:
            nunique : Series
        """
        axis = self._get_axis_number(axis) if axis is not None else 0
        return self._reduce_dimension(
            self._query_compiler.nunique(axis=axis, dropna=dropna)
        )