[WIP]`NLLB-MoE` Adds the moe model
# What does this PR do?
Fixes #21300
To-Dos:
- [x] Conversion script and original weights available [here](https://huggingface.co/ArthurZ/fairseq-nllb-moe)
- [x] Converted checkpoints and configuration file available:
        - [moe-128](https://huggingface.co/ArthurZ/nllb-moe-128) experts
- [x] Make the common tests go green
- [x] Implement top 2 gating mecanism
- [x] Add integration tests for:
    - [x] the routers
    - [x] the logits
    - [x] the generation using greedy search
  - [x] Cleanup the PR
_The documentation is not available anymore as the PR was closed or merged._