Add support for unknown scripts in NLLB

[ddaspit]

The NMT engine does not have good support for scripts that NLLB was not originally trained on. All characters would be added as individual tokens to the vocabulary resulting in poor translation quality. silnlp provides better support for unknown scripts. Here is an example of the silnlp config to enable this support:

  tokenizer:
    src_vocab_size: 0
    trained_tokens: true
    trg_vocab_size: 500
    update_src: true
    update_trg: true

When silnlp is configured in this way, it will train a new subword tokenizer on the target data using the specified trg_vocab_size. The new tokens are added to the NLLB vocabulary. This results in better translation quality for unknown scripts, then the default settings. Similar functionality should be added to the NMT engine. The engine could automatically detect when to enable this feature or it could be enabled using build options.

[ddaspit]

We probably can't automatically enable this feature right now. The best vocab size is currently determined by trial-and-error, so the EITL team will need to figure the best setting.