Fix #5974
Small bug introduced by the model outputs PR.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5999?src=pr&el=h1) Report
> Merging [#5999](https://codecov.io/gh/huggingface/transformers/pull/5999?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/76f52324b1e2d2bb631c80895a5f16ddc303a099&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `0.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5999/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5999?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #5999   +/-   ##
=======================================
  Coverage   78.66%   78.66%           
=======================================
  Files         146      146           
  Lines       26230    26229    -1     
=======================================
  Hits        20633    20633           
+ Misses       5597     5596    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5999?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/5999/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190cmFuc2ZvX3hsLnB5) | `79.12% <0.00%> (-0.04%)` | :arrow_down: |
| [src/transformers/generation\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5999/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3V0aWxzLnB5) | `97.11% <0.00%> (+0.28%)` | :arrow_up: |

------

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