code: def random_sent(self, index):
          
          t1, t2 = self.get_corpus_line(index)
          if random.random() > 0.5:
              label = 0
          else:
              t2 = self.get_random_line()
              label = 1
  
          assert len(t1) > 0
          assert len(t2) > 0
          return t1, t2, label
docs: Get one sample from corpus consisting of two sentences. With prob. 50% these are two subsequent sentences
          from one doc. With 50% the second sentence will be a random one from another doc.
          :param index: int, index of sample.
          :return: (str, str, int), sentence 1, sentence 2, isNextSentence Label