RoBERTa convert-script modified to support mapping of bpe tokens
# Reordering of embeddings possible when converting roberta with dict file

When RoBERTa is trained using fairseq the preprocessing pipeline bpe encodes tokens and stores a mapping of token ids to bpe-token ids in a dict.txt file. This is mostly just a re-ordering of the tokens (potentially with a few missing ones if they were not found in the training data). The earlier version of the conversion script maps the fairseq model directly creating a need for downstream processing to recover the original tokens, this removes that need by allowing a shuffling of the embedding tensors using the dict.txt file. This enables e.g. the "fill-mask" pipeline out of the box on a converted model.

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- [ ] Did you write any new necessary tests? 


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@julien-c (since prominent in blame)
 
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