Allocates a range of IDs in the datastore such that those IDs will not
be automatically assigned to new entities. You can only allocate IDs
for model keys from your app. If there is an error, raises a subclass of
Model instance, Key or string to serve as a template specifying the
ID sequence in which to allocate IDs. Returned ids should only be used
in entities with the same parent (if any) and kind as this key.