Fix mismatch of shape restriction in DrawBoundingBoxes

In the kernel of DrawBoundingBoxes, the shape of the input
images should be 4-D. Though in the shape function,
at the end `UnchangedShapeWithRankAtLeast(c, 3)` was used instead
(at the beginning of the shape function the validation is
`WithRank(c->input(0), 4, &images)` which is correct).

This fix address the discrepancy by changing to `UnchangedShape`.

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