docstr: Returns a dict of useful diagnostics after finishing sampling process.
code: def diagnostics(self):
      """
      Returns a dict of useful diagnostics after finishing sampling process.
      """
      # NB: should be not None for multiprocessing works
      return {}
  
file_path: pyro/pyro/infer/mcmc/mcmc_kernel.py
imports: from abc import ABCMeta, abstractmethod
label_desc: Use image node_id 4 for calling the MCMCKernel obj's underlying member method code with example usage: obj.diagnostics() and returns: dict