Compute the plate multiplier for given shapes.
The first shape is compared to all other shapes (using NumPy broadcasting rules). All the elements which are non-unit in the first shape but 1 in all other shapes are multiplied together.
This method is used, for instance, for computing a correction factor for messages to parents: If this node has non-unit plates that are unit plates in the parent, those plates are summed. However, if the message has unit axis for that plate, it should be first broadcasted to the plates of this node and then summed to the plates of the parent. In order to avoid this broadcasting and summing, it is more efficient to just multiply by the correct factor. This method computes that factor. The first argument is the full plate shape of this node (with respect to the parent). The other arguments are the shape of the message array and the plates of the parent (with respect to this node).