Add float16 support for NonMaxSuppressionV{2,3,4}

This fix tries to address the issue raised in 20199 where
there was no float16 support for NonMaxSuppressionV2.
As NonMaxSuppressionV2 is the earlier versions of API
and there are newer versions of NonMaxSuppression:
NonMaxSuppressionV2, NonMaxSuppressionV3, NonMaxSuppressionV4,
This fix exposes the float16 support to all of the above.
(Note in the master the default version used is NonMaxSuppressionV3)

This fix fixes 20199.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>