Transformer-XL: Fixed returned outputs when using `return_tuple=True`
Fixes #5974

`mems` are returned again when using `return_tuple=True`.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5975?src=pr&el=h1) Report
> Merging [#5975](https://codecov.io/gh/huggingface/transformers/pull/5975?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ae67b2439fb15954bfd8f0fdf521cf1a650bafb9&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/5975/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5975?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #5975   +/-   ##
=======================================
  Coverage   78.51%   78.51%           
=======================================
  Files         146      146           
  Lines       26214    26214           
=======================================
+ Hits        20581    20582    +1     
+ Misses       5633     5632    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5975?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/5975/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190cmFuc2ZvX3hsLnB5) | `79.16% <0.00%> (ø)` | |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5975/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.46% <0.00%> (+0.25%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5975?src=pr&el=footer). Last update [ae67b24...ed8722d](https://codecov.io/gh/huggingface/transformers/pull/5975?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Was fixed in different PR: #5999 