wrong args name: n_gpu -> gpus
The correct argument name should be `gpus`.

The change fixed the following error:
```bash
  warnings.warn(*args, **kwargs)
Traceback (most recent call last):                                                                                                                        
  File "/home/.conda/envs/hf/lib/python3.6/site-packages/pytorch_lightning/utilities/parsing.py", line 114, in __getattr__
    return self[key]
KeyError: 'n_gpu'
```
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5858?src=pr&el=h1) Report
> Merging [#5858](https://codecov.io/gh/huggingface/transformers/pull/5858?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/615be03f9d961c0c9722fe10e7830e011066772e&el=desc) will **decrease** coverage by `0.20%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5858/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5858?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5858      +/-   ##
==========================================
- Coverage   78.66%   78.46%   -0.21%     
==========================================
  Files         146      146              
  Lines       26200    26200              
==========================================
- Hits        20611    20558      -53     
- Misses       5589     5642      +53     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5858?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5858/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `26.02% <0.00%> (-69.52%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5858/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `84.96% <0.00%> (-1.76%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5858/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `98.79% <0.00%> (+33.89%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5858?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5858?src=pr&el=footer). Last update [615be03...713f748](https://codecov.io/gh/huggingface/transformers/pull/5858?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>great catch! was fixed on master by #5798 so I don't think we need this. Let us know if further issues!