MBartTokenizerTrimmed to support truncated embeddings
Motivation:
The embeddings table for MBART is huge, but only ~40K of the entries are used/finetuned for most wmt tasks. See https://github.com/pytorch/fairseq/issues/2120 

- needs vocab.json (fairseq Dictionary)
- needs to call `encode_as_pieces` with restricted vocabulary.

I will take this.
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