Generate help string with contents of registry.

def help_string():
  """Generate help string with contents of registry."""
  help_str = """
Registry contents:
------------------

  Models:
%s

  HParams:
%s

  RangedHParams:
%s

  Problems:
%s

  Optimizers:
%s

  Attacks:
%s

  Attack HParams:
%s

  Pruning HParams:
%s

  Pruning Strategies:
%s

  Env Problems:
%s
"""
  lists = tuple(
      display_list_by_prefix(entries, starting_spaces=4) for entries in [  # pylint: disable=g-complex-comprehension
          list_models(),
          list_hparams(),
          list_ranged_hparams(),
          list_base_problems(),
          list_optimizers(),
          list_attacks(),
          list_attack_params(),
          list_pruning_params(),
          list_pruning_strategies(),
          list_env_problems(),
      ])
  return help_str % lists