class_docstr: A vocab for the output tag sequence. 
class_docstr_tok: ["A","vocab","for","the","output","tag","sequence","."]
code: def build_vocab(self):
      counter = Counter(
          [w[self.idx] for sent in self.data for w in sent]
      )
  
      self._id2unit = VOCAB_PREFIX + list(
          sorted(
              list(counter.keys()),
              key=lambda k: counter[k],
              reverse=True,
          )
      )
      self._unit2id = {w: i for i, w in enumerate(self._id2unit)}
  
file_path: stanza/stanza/models/ner/vocab.py
imports: from collections import Counter, OrderedDict
  from stanza.models.common.vocab import BaseVocab, BaseMultiVocab, CharVocab, CompositeVocab
  from stanza.models.common.vocab import VOCAB_PREFIX
  from stanza.models.common.pretrain import PretrainedWordVocab
  from stanza.models.pos.vocab import WordVocab
label_desc: Use image node_id 1 for calling the TagVocab obj's underlying member method code with example usage: obj.build_vocab() without return types