simplify polbert usage example with pipelines
Indeed this is much simpler now! Pipelines look great :) 
Squashed into #3248 <|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3241?src=pr&el=h1) Report
> Merging [#3241](https://codecov.io/gh/huggingface/transformers/pull/3241?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/a4c75f149269099a98613f51b76cd0b579a109ee?src=pr&el=desc) will **increase** coverage by `0.18%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3241/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/3241?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3241      +/-   ##
==========================================
+ Coverage   77.82%   78.01%   +0.18%     
==========================================
  Files          98       98              
  Lines       16665    16665              
==========================================
+ Hits        12970    13001      +31     
+ Misses       3695     3664      -31
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3241?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3241/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `93.56% <0%> (-0.28%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3241/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.37% <0%> (+5.9%)` | :arrow_up: |

------

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