Get one sample from corpus consisting of a pair of two subsequent lines from the same doc.
        :param item: int, index of sample.
        :return: (str, str), two subsequent sentences from corpus

def get_corpus_line(self, item):
        """
        Get one sample from corpus consisting of a pair of two subsequent lines from the same doc.
        :param item: int, index of sample.
        :return: (str, str), two subsequent sentences from corpus
        """
        t1 = ""
        t2 = ""
        assert item < self.corpus_lines
        if self.on_memory:
            sample = self.sample_to_doc[item]
            t1 = self.all_docs[sample["doc_id"]][sample["line"]]
            t2 = self.all_docs[sample["doc_id"]][sample["line"]+1]
            # used later to avoid random nextSentence from same doc
            self.current_doc = sample["doc_id"]
            return t1, t2
        else:
            if self.line_buffer is None:
                # read first non-empty line of file
                while t1 == "" :
                    t1 = next(self.file).strip()
                    t2 = next(self.file).strip()
            else:
                # use t2 from previous iteration as new t1
                t1 = self.line_buffer
                t2 = next(self.file).strip()
                # skip empty rows that are used for separating documents and keep track of current doc id
                while t2 == "" or t1 == "":
                    t1 = next(self.file).strip()
                    t2 = next(self.file).strip()
                    self.current_doc = self.current_doc+1
            self.line_buffer = t2

        assert t1 != ""
        assert t2 != ""
        return t1, t2