[GraphBolt] items are not shuffled across the whole set if `num_workers>0`

[Rhett-Ying]

🔨Work Item

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Project tracker: https://github.com/orgs/dmlc/projects/2

Description

split item before shuffle results in significant accuracy drop. We should shuffle across the whole set first, then split items among workers. It's worth checking if torch.DL shuffle in this way.

buffer_size of ItemShufflerAndBatcher could always be len(item_set) to simplify the code logic.

Depending work items or issues

[Rhett-Ying]

For ItemSampler , it's fixed in https://github.com/dmlc/dgl/pull/6982. One side effect of this fix is all workers are using same seed generator for DistributedItemSampler's shuffle.