Infer highest epoch number from file names in data_dir.

def infer_last_epoch_num(data_dir):
  """Infer highest epoch number from file names in data_dir."""
  names = os.listdir(data_dir)
  epochs_str = [re.findall(pattern=r".*\.(-?\d+)$", string=name)
                for name in names]
  epochs_str = sum(epochs_str, [])
  return max([int(epoch_str) for epoch_str in epochs_str])