[Benchmarks] improve Example Plotter
This PR make it possible to plot csv files that have "N/A" values.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5245?src=pr&el=h1) Report
> Merging [#5245](https://codecov.io/gh/huggingface/transformers/pull/5245?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/9fe09cec76efa1e221c3fd6eb8520ba0a911f092&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5245/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5245?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5245      +/-   ##
==========================================
- Coverage   77.93%   77.92%   -0.01%     
==========================================
  Files         138      138              
  Lines       23860    23859       -1     
==========================================
- Hits        18595    18593       -2     
- Misses       5265     5266       +1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5245?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/benchmark/benchmark\_args\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5245/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9iZW5jaG1hcmsvYmVuY2htYXJrX2FyZ3NfdXRpbHMucHk=) | `89.13% <ø> (-0.24%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5245/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `86.00% <0.00%> (-0.15%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5245?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/5245?src=pr&el=footer). Last update [9fe09ce...882b09d](https://codecov.io/gh/huggingface/transformers/pull/5245?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
