Returns get_attr_fn(distribution) when vertexes take on values.

vertexes A nest structrue of lazybones deferred random variable. Usually this is a value representation (e.g., a random sample, the mean) of a Deferred wrapped distribution-like object.
values A nest structrue of Array.
get_attr_fn Callable to get a Class attribute from a distribution-like object that used to evaluate on the vertexes itself (e.g., logpdf or pdf from a scipy.stats distributions).
combine Method to combine the distribution measure (e.g., sum to combine logpdf, prod to combine pdf).
reduce_op Optional computation to reduce the distribution measure for each vertexes, default to identity.