[Benchmark] fix benchmark non standard model
This PR fixes a typo in Benchmark, that allows to load specifc architectures and not just the base model.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5801?src=pr&el=h1) Report
> Merging [#5801](https://codecov.io/gh/huggingface/transformers/pull/5801?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/8ce610bc96ace8be6b514c0f2c3fb0f76eb7ee15&el=desc) will **increase** coverage by `0.08%`.
> The diff coverage is `75.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5801/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5801?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5801      +/-   ##
==========================================
+ Coverage   77.33%   77.42%   +0.08%     
==========================================
  Files         146      146              
  Lines       26047    26047              
==========================================
+ Hits        20143    20166      +23     
+ Misses       5904     5881      -23     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5801?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/benchmark/benchmark\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/5801/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9iZW5jaG1hcmsvYmVuY2htYXJrX3RmLnB5) | `65.03% <50.00%> (+3.49%)` | :arrow_up: |
| [src/transformers/benchmark/benchmark.py](https://codecov.io/gh/huggingface/transformers/pull/5801/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9iZW5jaG1hcmsvYmVuY2htYXJrLnB5) | `81.10% <100.00%> (+7.08%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5801/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9vcGVuYWkucHk=) | `20.27% <0.00%> (-74.92%)` | :arrow_down: |
| [src/transformers/tokenization\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/5801/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtLnB5) | `16.26% <0.00%> (-66.67%)` | :arrow_down: |
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/5801/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `28.34% <0.00%> (-6.55%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5801/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `83.70% <0.00%> (-2.76%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5801/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `81.02% <0.00%> (-1.29%)` | :arrow_down: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5801/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `89.95% <0.00%> (-0.81%)` | :arrow_down: |
| ... and [1 more](https://codecov.io/gh/huggingface/transformers/pull/5801/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5801?src=pr&el=footer). Last update [8ce610b...dbf09d1](https://codecov.io/gh/huggingface/transformers/pull/5801?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
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