Fix roberta checkpoint conversion script
After #2521 and #2958, this script stopped working. We need to set the bias on the new `decoder` Linear directly.

cc @LysandreJik
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3642?src=pr&el=h1) Report
> Merging [#3642](https://codecov.io/gh/huggingface/transformers/pull/3642?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/1789c7daf1b8013006b0aef6cb1b8f80573031c5&el=desc) will **increase** coverage by `0.97%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3642/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3642?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3642      +/-   ##
==========================================
+ Coverage   77.32%   78.29%   +0.97%     
==========================================
  Files         104      104              
  Lines       17628    17628              
==========================================
+ Hits        13630    13801     +171     
+ Misses       3998     3827     -171     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3642?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3642/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `93.45% <0.00%> (+0.81%)` | :arrow_up: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/3642/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `81.54% <0.00%> (+1.34%)` | :arrow_up: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/3642/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `75.77% <0.00%> (+2.29%)` | :arrow_up: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/3642/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `98.24% <0.00%> (+2.63%)` | :arrow_up: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/3642/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `95.71% <0.00%> (+10.00%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3642/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9weXRvcmNoX3V0aWxzLnB5) | `89.93% <0.00%> (+81.20%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3642?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/3642?src=pr&el=footer). Last update [1789c7d...bd60e83](https://codecov.io/gh/huggingface/transformers/pull/3642?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Hi @myleott, thanks for looking into this! Indeed the conversion script is failing due to that bias. The checkpoints on the S3 do not need to be re-uploaded, it was only the conversion of the new checkpoints that needed to be updated.

I manually checked that we have the same results between the torch hub models and those hosted on our S3 + we have [integration tests that test just that ](https://github.com/huggingface/transformers/blob/master/tests/test_modeling_roberta.py#L322) :)

Thanks for your fix! 