[BUG] IntervalIndex.unique is resulting in an incomplete due to lack of post-processing

[galipremsagar]

Describe the bug
Calling unique on an IntervalColumn(which is a StructColumn) is resulting in incomplete creation of the actual IntervalColumn, thus leaving out the StructColumn as is.

Steps/Code to reproduce bug

In [10]: i = cudf.IntervalIndex.from_breaks([0, 1, 2, 3])

In [11]: i
Out[11]: IntervalIndex([(0, 1], (1, 2], (2, 3]], dtype='interval')

In [12]: i.unique()
Out[12]: IntervalIndex([{'0': 0, '1': 1}, {'0': 1, '1': 2}, {'0': 2, '1': 3}], dtype='struct')

Expected behavior

In [11]: i.unique()
Out[11]: IntervalIndex([(0, 1], (1, 2], (2, 3]], dtype='interval')

Environment overview (please complete the following information)

• Environment location: [Bare-metal]
• Method of cuDF install: [from source]