fix mirrored pair input

[HaokunLiu]

When discussion with Phil about the preprocess function, I found I wrote a typo when implementing the preprocessing of gpt2-like single directional model.

This will have slight negatively affect on the performance of gpt, gpt2 and transformer-xl on sts-b, mrpc, qqp. But not other transformer models or other tasks.

[pyeres]

Confirmed that this change reorders inputs in line with what's described in GPT (section 3.3).