Update pipelines.py
Modified QA pipeline to consider all features for each example before generating topk answers. 
Current pipeline only takes one SquadExample, one SquadFeature, one start logit list, one end logit list to retrieve the answer, this is not correct as one SquadExample can produce multiple SquadFeatures.
Hi @Perseus14, thanks for your contribution :).

I took the liberty to apply black formatting so that tests are happy.

Looks good to me 👍 <|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2459?src=pr&el=h1) Report
> Merging [#2459](https://codecov.io/gh/huggingface/transformers/pull/2459?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/16ce15ed4bd0865d24a94aa839a44cf0f400ef50?src=pr&el=desc) will **increase** coverage by `0.14%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2459/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2459?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2459      +/-   ##
==========================================
+ Coverage   73.24%   73.39%   +0.14%     
==========================================
  Files          87       87              
  Lines       15001    15005       +4     
==========================================
+ Hits        10988    11013      +25     
+ Misses       4013     3992      -21
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2459?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/2459/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `69.03% <100%> (+0.35%)` | :arrow_up: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2459/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `88% <0%> (+0.16%)` | :arrow_up: |
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/2459/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `35.97% <0%> (+6.6%)` | :arrow_up: |

------

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> **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/2459?src=pr&el=footer). Last update [16ce15e...0d6c17f](https://codecov.io/gh/huggingface/transformers/pull/2459?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Ok great, thanks @Perseus14 @mfuntowicz!