re axis where NaNs will be
            filled.
        copy : bool, default True
            Whether to make a copy of the data before filling. If False, then
            the original should be modified and no new memory should be allocated.
            For ExtensionArray subclasses that cannot do this, it is at the
            author's discretion whether to ignore "copy=False" or to raise.

        Returns
        -------
        filled : IntervalArray with NA/NaN filled
        FN)