Bias should be resized with the weights
Created a link between the linear layer bias and the model attribute bias. This does not change anything for the user nor for the conversion scripts, but allows the `resize_token_embeddings` method to resize the bias as well as the weights of the decoder.

Added a test.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2521?src=pr&el=h1) Report
> Merging [#2521](https://codecov.io/gh/huggingface/transformers/pull/2521?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/6c32d8bb95aa81de6a047cca5ae732b93b9db020?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2521/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2521?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2521      +/-   ##
==========================================
+ Coverage   73.24%   73.25%   +<.01%     
==========================================
  Files          87       87              
  Lines       15008    15011       +3     
==========================================
+ Hits        10993    10996       +3     
  Misses       4015     4015
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2521?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2521/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `64.42% <100%> (+0.17%)` | :arrow_up: |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/2521/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `87.72% <100%> (+0.02%)` | :arrow_up: |
| [src/transformers/modeling\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/2521/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hbGJlcnQucHk=) | `78.91% <100%> (+0.05%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2521?src=pr&el=footer). Last update [6c32d8b...b7832ab](https://codecov.io/gh/huggingface/transformers/pull/2521?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
