When selecting Render As: Collection for particles right now you can either render the whole collection as a single instance OR a single object from the collection.
This patch adds support to render the child collections of the specified collection as individual instances for each vertex. This is probably best explained with a picture:
This patch also fixes a behavior when instancing whole collections (and child collections) where the global location of the duplicated objects would be kept as an offset for each particle, forcing users to center the collection at the world origin. I've also adjusted the code so the object list is not allocated when instancing whole collections as this was an unneeded allocation.