docs: def validate_holdout_selfplay():
      """Validate on held-out selfplay data."""
      holdout_dirs = (os.path.join(fsdb.holdout_dir(), d)
                      for d in reversed(gfile.ListDirectory(fsdb.holdout_dir()))
                      if gfile.IsDirectory(os.path.join(fsdb.holdout_dir(), d))
                      for f in gfile.ListDirectory(os.path.join(fsdb.holdout_dir(), d)))
  
      # This is a roundabout way of computing how many hourly directories we need
      # to read in order to encompass 20,000 holdout games.
      holdout_dirs = set(itertools.islice(holdout_dirs), 20000)
      cmd = ['python3', 'validate.py'] + list(holdout_dirs) + [
          '--use_tpu',
          '--tpu_name={}'.format(TPU_NAME),
          '--flagfile=rl_loop/distributed_flags',
          '--expand_validation_dirs']
      mask_flags.run(cmd)