[Fix] Checkpoint saving and I/O for XLNetQASimple 
**1.  [Fix] Create directories for model checkpoints**

Saving model checkpoints without making directories would lead to error. 
(i.e.  The output directory will not exist using the currenct script.)

The [older version](https://github.com/huggingface/transformers/blob/3f5ccb183e3cfa755dea2dd2afd9abbf1a0f93b8/examples/run_squad.py#L193) of the part of this script works fine.

**2.  [Fix] Fix wrong input/output structures for XLNet**

Running XLNet using the current script would lead to this error below.
> TypeError: forward() got an unexpected keyword argument 'cls_index'

The reason is that the [AutoModelForQuestionAnswering](https://github.com/huggingface/transformers/blob/ba2400189b2242620868096ae49babf93bd9ce00/examples/question-answering/run_squad.py#L735) would initialize XLNetForQuestionAnsweringSimple, while the I/O structures are written for XLNetForQuestionAnswering.
This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
