[{m}bart] Fix final_logits bias warning
This warning is safe to ignore, but can be easily fixed.

```bash
Some weights of BartForConditionalGeneration were not initialized from the model checkpoint at facebook/mbart-large-en-ro and are newly initialized: ['final_logits_bias']
You should probably TRAIN this model on a down-stream task to be able to use it for predictions and inference.
```
also
```bash
Some weights of the model checkpoint at facebook/mbart-large-en-ro were not used when initializing BartForConditionalGeneration: ['lm_head.weight']
```<|||||>Hey @sshleifer 
Want me to pick this one up?<|||||>It's non trivial. You probably have to update S3. I would pick another.<|||||>Maybe try to understand why 
```
tests/test_pipelines.py::MonoColumnInputTestCase::test_torch_fill_mask_results
```
has started failing, half of #5265 