Fix CTRL past
Fixes the issue with the re-usable past in CTRL.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1740?src=pr&el=h1) Report
> Merging [#1740](https://codecov.io/gh/huggingface/transformers/pull/1740?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/5afca00b4732f57329824e1538897e791e02e894?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1740/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1740?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1740      +/-   ##
==========================================
+ Coverage   84.06%   84.06%   +<.01%     
==========================================
  Files         105      105              
  Lines       15536    15537       +1     
==========================================
+ Hits        13060    13061       +1     
  Misses       2476     2476
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1740?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/1740/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX2N0cmwucHk=) | `96.47% <100%> (+0.01%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1740?src=pr&el=footer). Last update [5afca00...8da47b0](https://codecov.io/gh/huggingface/transformers/pull/1740?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Ok great, nice catch and fix!