Can't instantiate abstract class JiantBartModel with abstract methods normalize_tokenizations

[Chang-Hongyang]

This error is reported with BART, but not with BERT

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To Reproduce

1. Tell use which version of jiant you're using
2. Describe the environment where you're using jiant, e.g, "2 P40 GPUs"
3. Provide the experiment config artifact (e.g., defaults.conf)

Expected behavior
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