Improve shape function of NonMaxSuppression (#16890)

* Improve shape function of NonMaxSuppression

In the docs for `tf.image.non_max_suppression`, the shapes
of the args `boxes` and `scores` are `[num_boxes, 4]` and `[num_boxes]`
respectively.

This fix improve the shape function of NonMaxSuppression so that
`boxes_shape[0] = scores_shape[0] = num_boxes`.

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

* Add additional test case for shape function of NonMaxSuppression

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