Get alphabet and label names, union across all dirs.

def _get_names(dirs):
  """Get alphabet and label names, union across all dirs."""
  alphabets = set()
  label_names = {}
  for d in dirs:
    for example in _walk_omniglot_dir(d):
      alphabet, alphabet_char_id, label, _ = example
      alphabets.add(alphabet)
      label_name = "%s_%d" % (alphabet, alphabet_char_id)
      if label in label_names:
        assert label_names[label] == label_name
      else:
        label_names[label] = label_name
  label_names = [label_names[k] for k in sorted(label_names)]
  return alphabets, label_names