code: def posterior(self, obs):
      concentration = self._latent.concentration
      rate = self._latent.rate
      reduce_dims = len(obs.size()) - len(rate.size())
      num_obs = obs.shape[:reduce_dims].numel()
      summed_obs = sum_leftmost(obs, reduce_dims)
      return dist.Gamma(concentration + summed_obs, rate + num_obs)
  
imports: from collections import defaultdict
  import torch
  import pyro.distributions
  from pyro import poutine
  from pyro.distributions.util import sum_leftmost
  from pyro.poutine.messenger import Messenger
  from pyro.poutine.util import site_is_subsample
label_desc: Use image node_id 4 for calling the GammaPoissonPair obj's underlying member method code with example usage: obj.posterior(obs) and returns: dist