Change back pipeline signatures
As discussed with @julien-c in the merged #3055.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3105?src=pr&el=h1) Report
> Merging [#3105](https://codecov.io/gh/huggingface/transformers/pull/3105?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/b31f7150190cdf13950607f8ee1efe11b352c909?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3105/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/3105?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3105      +/-   ##
==========================================
+ Coverage    77.6%   77.62%   +0.02%     
==========================================
  Files          98       98              
  Lines       16250    16230      -20     
==========================================
- Hits        12611    12599      -12     
+ Misses       3639     3631       -8
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3105?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/3105/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `70.95% <ø> (+0.52%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3105?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/3105?src=pr&el=footer). Last update [b31f715...da52b39](https://codecov.io/gh/huggingface/transformers/pull/3105?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
