[Flax] Add Flax implementation of `BlenderbotSmallModel`
# 🚀 Feature request

Add Flax implementation of `BlenderbotSmall` models.

## Motivation

Improve the level of Flax/Jax support.

## Your contribution

I'll be happy to do it once the problems in #13633 will be resolved and the PR will be merged.

@patrickvonplaten 
This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>waiting for #13633 