Enable HF nllb conversion

[francoishernandez]

NLLB support has been unclear since switching from OpenNMT to Eole.
This PR facilitates the conversion of official HF NLLB models (e.g. https://huggingface.co/facebook/nllb-200-distilled-1.3B).
This should also facilitate re-enabling support of pre-trained seq2seq models such as BART or T5.

That being said, the current structure of convert_HF needs to be reviewed to better support the encoder/decoder duality.
#156 was a first major step in making convert_HF, more modular, and #153 introduced the support of encoder keys, but now we need to meld all this into a more robust logic.
Also, we should probably define a better "HF settings deduction waterfall" in build_config_dict, but I'm not sure there is any centralized repository of all the possible values, as they are defined per model.

Note: HF tokenization was not tested yet, there might be some edge case to tackle with the prefix transform.
EDIT: HF tokenization patched in 3fd59e6