    def load_dataset(self, split, epoch=1, combine=False, **kwargs):
        """Load a given dataset split.

        Args:
            split (str): name of the split (e.g., train, valid, test)
        """
        paths = self.cfg.data.split(":")
        assert len(paths) > 0
        data_path = paths[(epoch - 1) % len(paths)]
        split_path = os.path.join(data_path, split)

        if self.cfg.langs is None:
            languages = sorted(
                [
                    name
                    for name in os.listdir(data_path)
                    if os.path.isdir(os.path.join(data_path, name))
                ]
            )
        else:
            languages = self.cfg.langs.split(",")
            for name in languages:
                p = os.path.join(data_path, name)
                assert os.path.exists(p), "data not found: {}".format(p)

        logger.info("Training on {0} languages: {1}".format(len(languages), languages))
        logger.info(
            "Language to id mapping: ", {lang: id for id, lang in enumerate(languages)}
        )

        mask_whole_words = get_whole_word_mask(self.cfg.bpe, self.dictionary)
        language_without_segmentations = self.cfg.no_whole_word_mask_langs.split(",")
        lang_datasets = []
        for language in languages:
            split_path = os.path.join(data_path, language, split)

            dataset = data_utils.load_indexed_dataset(
                split_path,
                self.source_dictionary,
                self.cfg.dataset_impl,
                combine=combine,
            )
            if dataset is None:
                raise FileNotFoundError(
                    "Dataset not found: {} ({})".format(split, split_path)
                )

            end_token = (
                self.source_dictionary.index("[{}]".format(language))
                if self.cfg.add_lang_token
                else self.source_dictionary.eos()
            )

            # create continuous blocks of tokens
            dataset = TokenBlockDataset(
                dataset,
                dataset.sizes,
                self.cfg.tokens_per_sample - 2,  # one less for <s>
                pad=self.source_dictionary.pad(),
                eos=end_token,
                break_mode=self.cfg.sample_break_mode,
            )
            logger.info("loaded {} blocks from: {}".format(len(dataset), split_path))

            # prepend beginning-of-sentence token (<s>, equiv. to [CLS] in BERT)
            dataset = PrependTokenDataset(dataset, self.source_dictionary.bos())
            dataset = AppendTokenDataset(dataset, end_token)

            lang_mask_whole_words = (
                mask_whole_words
                if language not in language_without_segmentations
                else None
            )
            lang_dataset = DenoisingDataset(
                dataset,
                dataset.sizes,
                self.dictionary,
                self.mask_idx,
                lang_mask_whole_words,
                shuffle=self.cfg.shuffle_instance,
                seed=self.cfg.seed,
                mask=self.cfg.mask,
                mask_random=self.cfg.mask_random,
                insert=self.cfg.insert,
                rotate=self.cfg.rotate,
                permute_sentences=self.cfg.permute_sentences,
                bpe=self.cfg.bpe,
                replace_length=self.cfg.replace_length,
                mask_length=self.cfg.mask_length,
                poisson_lambda=self.cfg.poisson_lambda,
                eos=None
                if not self.cfg.add_lang_token
                else self.source_dictionary.index("[{}]".format(language)),
            )
            lang_datasets.append(lang_dataset)

        dataset_lengths = np.array(
            [len(d) for d in lang_datasets],
            dtype=float,
        )
        logger.info(
            "loaded total {} blocks for all languages".format(
                int(dataset_lengths.sum()),
            )
        )
        if split == self.cfg.train_subset:
            # For train subset, additionally up or down sample languages.
            sample_probs = self._get_sample_prob(dataset_lengths)
            logger.info(
                "Sample probability by language: {}".format(
                    {
                        lang: "{0:.4f}".format(sample_probs[id])
                        for id, lang in enumerate(languages)
                    }
                )
            )
            size_ratio = (sample_probs * dataset_lengths.sum()) / dataset_lengths
            logger.info(
                "Up/Down Sampling ratio by language: {}".format(
                    {
                        lang: "{0:.2f}".format(size_ratio[id])
                        for id, lang in enumerate(languages)
                    }
                )
            )

            resampled_lang_datasets = [
                ResamplingDataset(
                    lang_datasets[i],
                    size_ratio=size_ratio[i],
                    seed=self.cfg.seed,
                    epoch=epoch,
                    replace=size_ratio[i] >= 1.0,
                )
                for i, d in enumerate(lang_datasets)
            ]
            dataset = ConcatDataset(
                resampled_lang_datasets,
            )
        else:
            dataset = ConcatDataset(lang_datasets)
            lang_splits = [split]
            for lang_id, lang_dataset in enumerate(lang_datasets):
                split_name = split + "_" + languages[lang_id]
                lang_splits.append(split_name)
                self.datasets[split_name] = lang_dataset

            if split in self.cfg.valid_subset:
                self.cfg.valid_subset = self.cfg.valid_subset.replace(
                    split, ",".join(lang_splits)
                )

        with data_utils.numpy_seed(self.cfg.seed + epoch):
            shuffle = np.random.permutation(len(dataset))

        self.datasets[split] = SortDataset(
            dataset,
            sort_order=[
                shuffle,
                dataset.sizes,
            ],
        )
Load a given dataset split.

Args:
split (str): name of the split (e.g., train, valid, test)
fairseq/tasks/multilingual_denoising.py