Instead of using input.get_shape() fonction
We can pass input image shape to yolo_v3 and yolo_v3_tiny constructor
function.
Since the input placeholder shape is statically defined (to None, size, size, 3)
We can have access to this 'size' when constructing the yolo_v3 or
yolo_v3_tiny models.

This if more efficient for inference.
This commit should not break any anterior codes since it is only adding
1 optional argument to model constructor functions

There was a bug in the way the all zero prediction was removed from the
image_prediction tensor

This solution is correct, much more simple and clear.