Add tie_weights to XLNetForSequenceClassification
XLNetForSequenceClassification doesn't have tie_weights() but initialization will call it, or we can made a function in XLNetPretrainedModel?
# [Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/716?src=pr&el=h1) Report
> Merging [#716](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/716?src=pr&el=desc) into [xlnet](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/commit/c946bb51a61f67b0c9eaae1c9cf6f164a7748e37?src=pr&el=desc) will **increase** coverage by `0.03%`.
> The diff coverage is `50%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/716/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/716?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##            xlnet     #716      +/-   ##
==========================================
+ Coverage   62.18%   62.22%   +0.03%     
==========================================
  Files          22       22              
  Lines        4742     4744       +2     
==========================================
+ Hits         2949     2952       +3     
+ Misses       1793     1792       -1
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/716?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_pretrained\_bert/modeling\_xlnet.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/716/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvbW9kZWxpbmdfeGxuZXQucHk=) | `65.16% <50%> (-0.06%)` | :arrow_down: |
| [pytorch\_pretrained\_bert/tokenization\_gpt2.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/716/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvdG9rZW5pemF0aW9uX2dwdDIucHk=) | `83.51% <0%> (+1.06%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/716?src=pr&el=footer). Last update [c946bb5...00547bd](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/716?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Thanks but we don't need that, the model was work in progress.