def collater(self, samples):
        """
        Since we enforce all datsets to be the same, collating is just
        picking the first one and doing collate.
        """
        if len(samples) == 0:
            return None

        return list(self.datasets.values())[0].collater(samples)

        Since we enforce all datsets to be the same, collating is just
        picking the first one and doing collate.
        
decoding/IAD/fairseq/fairseq/data/multi_corpus_dataset.py