[WIP]`NLLB-MoE` Adds the moe model

[ArthurZucker]

What does this PR do?

Fixes #21300 To-Dos:

• [x] Conversion script and original weights available here
• [x] Converted checkpoints and configuration file available: - 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

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.