words of overflowing_tokens in function truncate_sequences is not in right order
https://github.com/huggingface/transformers/blob/6b4c617666fd26646d44d54f0c45dfe1332b12ca/src/transformers/tokenization_utils_base.py#L2570
```
                   if not overflowing_tokens:
                        window_len = min(len(pair_ids), stride + 1)
                    else:
                        window_len = 1
                    overflowing_tokens.extend(pair_ids[-window_len:])
                    pair_ids = pair_ids[:-1]
```
In my understanding, the overflowing_tokens is a sub sequences of the second sequence (pair_ids). But in this code, the order of words in overflowing_tokens is not same as the pair_ids (and also different with the reversed sequence of pair_ids if window_len != 1).
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