Advice needed: Adding more FSMT models
# 🌟 New model addition

## Model description

I am planning to contribute a series of FSMT models to the model hub. The models have been trained for a paper that is currently under review.

Before working on a PR I wanted to ask for some advice:

### normalize_before
The new models have been trained with Fairseq's option `normalize_before=True`, while the existing FSMT implementation uses `normalize_before=False`. I understand that copy-pasting model code is preferred to extending the configuration. This would mean that a near-duplicate module `fsmt_prenorm` needs to be created. Is this correct?

### Adequate base branch
The FSMT module is currently being refactored (https://github.com/huggingface/transformers/pull/11218). Do you recommend that I start from the master branch or from the PR's feature branch, which is nearly completed?
@patil-suraj I am still very motivated to work on the pull request :)  Just let me know if you need more information to answer my question.

In case you're interested, the paper describing our models is now public (https://openreview.net/forum?id=RvO9DqoWI9V). I believe the models could be of value to others in the community.