Fix issue with Luong attention when scale=True and dtype of tf.float16/tf.float64 (#18106)

* Fix issue with Luong attention when scale=True and dtype=tf.float16/tf.float64

This fix tries to address the issue raised in 18099 where
Luong throws a ValueError when scale=True and dtype is not tf.float32.

This fix addresses the issue with the additional test case added.

This fix fixes 18099.

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

* Fix pylint issue

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

* Add test case for Luong attention with scale=True and dtype=float16/float64

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

* Add assertEqual to confirm the dtypes of the output

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