Improve shape function of NonMaxSuppression (#16664)

* Improve shape function of NonMaxSuppression

This fix tries to improve shape function of NonMaxSuppression.
As was specified in the docs, the shapes of parameters of
`tf.image.non_max_suppression` are clearly defined with:
boxes: 2-D with shape [num_boxes, 4]
scores: 1-D with shape [num_boxes]
max_output_size: 0-D scalar
iou_threshold: 0-D scalar

However, there is no shape check in the shape function of
NonMaxSuppression.

This fix adds the shape check for NonMaxSuppression,
and adds additinal test cases for it.

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

* Add additional test cases for shape check of NonMaxSuppression.

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