feat: use datasets.IterableDataset shard if possible (#3635)
* feat: use datasets.IterableDataset shard if possible.
When `accelerator.prepare` is called on a
`datasets.IterableDataset`, use the `shard` method to
split the dataset across the available processes. This
allows for more efficient data loading and processing.
Without load and slice overhead of `IterableDatasetShard`
* dataset
* remove unused import
* style
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Co-authored-by: wuwenxu.01 <wuwenxu.01@bytedance.com>
