Initalize symbol table dictionary

        :param sentences:
        :param markers:
        :return:

def _build_vocab(self, sentences, markers=[]):
        """
        Initalize symbol table dictionary

        :param sentences:
        :param markers:
        :return:

        """
        from snorkel.learning.pytorch.rnn.utils import SymbolTable

        vocab = Counter()
        for sent in sentences:
            for w in sent:
                vocab[w] += 1
        word_dict = SymbolTable()
        list(map(word_dict.get, vocab))
        list(map(word_dict.get, markers))
        return word_dict