missing reference `from model_bertabs import BertAbsSummarizer`
[This line](https://github.com/huggingface/transformers/blob/1a779ad7ecb9e5215b6bd1cfa0153469d37e4274/examples/seq2seq/bertabs/convert_bertabs_original_pytorch_checkpoint.py#L28) reads

> from model_bertabs import BertAbsSummarizer

yet neither `model_bertabs` not `BertAbsSummarizer` can be found in the repository
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