add explaining example to XLNet LM modeling
1) adds example explaining how XLNet could be used for standard auto-regressive modelling.
2) puts add_special_tokens=True for simple example to make sure no <sep> and <cls> are added to input. The are used for special training (similar to BERT) and might be confusing to user
> Cool! Indeed, if I remember the paper correctly there is no need to shift the labels. Maybe we could update the [associated docstring](https://github.com/huggingface/transformers/blob/master/src/transformers/modeling_xlnet.py#L980-L985) to make sure no one gets confused again?

Done :-) <|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2997?src=pr&el=h1) Report
> Merging [#2997](https://codecov.io/gh/huggingface/transformers/pull/2997?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/21d8b6a33ebf96680b6a0aabd27fa7eaa068da93?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2997/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2997?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2997   +/-   ##
=======================================
  Coverage   77.19%   77.19%           
=======================================
  Files          98       98           
  Lines       16013    16013           
=======================================
  Hits        12361    12361           
  Misses       3652     3652
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2997?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/2997/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `75.77% <ø> (ø)` | :arrow_up: |

------

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