Add problem hparams for the problems.

def add_problem_hparams(hparams, problem_name_or_instance):
  """Add problem hparams for the problems."""
  if isinstance(problem_name_or_instance, problem_lib.Problem):
    problem = problem_name_or_instance
  else:
    problem = registry.problem(problem_name_or_instance)
  p_hparams = problem.get_hparams(hparams)
  hparams.problem = problem
  hparams.problem_hparams = p_hparams