Mel lain processor modify

I think this std::find is wrong here --- it is slow, and it should not be necessary.
You probably need this:

int (token_index = 0; token_index < field.token_id_size(); ++token_index) {
  int token_id = field.token_id(token_index);
  int token_count = field.token_count(token_index);
  n_dw(token_id, item_index) += token_count;
}