Validate ignore_index type in drop_duplicates

[mroeschke]

Description

Currently allows odd behavior like

In [1]: import cudf

In [4]: df = cudf.DataFrame({"a": [1, 2, 1, 3]})

In [6]: df.drop_duplicates(ignore_index="True")
Out[6]: 
   a
0  1
1  2
2  3

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[galipremsagar]

/merge