Compute outer box encloses an object with a margin.

boxes a tensor whose last dimension is 4 representing the coordinates of boxes in ymin, xmin, ymax, xmax order.
image_shape a list of two integers, a two-element vector or a tensor such that all but the last dimensions are broadcastable to boxes. The last dimension is 2, which represents [height, width].
scale a float number specifying the scale of output outer boxes to input boxes.

outer_boxes a tensor whose shape is the same as boxes representing the outer boxes.