code: def backfill_unk_emb(self, E, filled_words):
          
          unk_emb = E[self[self._unk]]
          for i, word in enumerate(self):
              if word not in filled_words:
                  E[i] = unk_emb
docs: Backfills an embedding matrix with the embedding for the unknown token.
  
          :param E: original embedding matrix of dimensions `(vocab_size, emb_dim)`.
          :param filled_words: these words will not be backfilled with unk.
  
          NOTE: this function is for internal use.