bugfix: pass on tokenizer to pipeline in load_graph_from_args
I think I found a small bug in the `load_graph_from_args` function in `convert_graph_to_onnx.py` as it accepts a tokenizer as input but doesn't pass it on to the pipeline inside the function.

Love the library 🤗
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4489?src=pr&el=h1) Report
> Merging [#4489](https://codecov.io/gh/huggingface/transformers/pull/4489?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/14cb5b35faeda7881341656aacf89d12a8a7e07b&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/4489/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4489?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4489      +/-   ##
==========================================
- Coverage   78.04%   78.03%   -0.01%     
==========================================
  Files         123      123              
  Lines       20477    20477              
==========================================
- Hits        15981    15980       -1     
- Misses       4496     4497       +1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4489?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4489/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.44% <0.00%> (-0.42%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4489?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/4489?src=pr&el=footer). Last update [14cb5b3...a5ce320](https://codecov.io/gh/huggingface/transformers/pull/4489?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Indeed ! Thanks for spotting this @RensDimmendaal 