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

## 🔨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