Just do the partitioning/shuffling in the local leader worker.

[knighton]

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[karan6181]

For tracking purpose: As discussed offline, please change this line https://github.com/mosaicml/streaming/blob/main/streaming/base/dataset.py#L156 from hasattr(os.environ, 'RANK') to 'RANK' in os.environ and we will no longer see the FileNotFoundError: [Errno 2] No such file or directory: '/tmp/mds-cache/mds-c4/train/index.json' error.