[Performance] Optimize the use of alternative streams in dataloader

[yaox12]

Description

Main changes:

1. Use a fixed alternative stream for H2D copy in the dataloader instead of creating a new stream every time. H2D copy to the same GPU in different streams won't overlap.
2. Put input nodes, seed nodes, and subgraph transfer in the alternative stream.

Before, only node/edge features transfer can overlap with the last step's computing kernels, while the subgraph transfer is not in the alternative stream. There is a launch gap between the H2D copy and the following computing kernels.

With this PR, all data transfer in the dataloader can overlap with computation, and the launch gap is reduced.

The timeline comes from RGCN training on MAG240M.

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
• Changes are complete (i.e. I finished coding on this PR)
• To the best of my knowledge, examples are either not affected by this change,
  or have been fixed to be compatible with this change

[dgl-bot]

To trigger regression tests:

• @dgl-bot run [instance-type] [which tests] [compare-with-branch];
  For example: @dgl-bot run g4dn.4xlarge all dmlc/master or @dgl-bot run c5.9xlarge kernel,api dmlc/master

[dgl-bot]

Commit ID: 817be8a

Build ID: 5

Status: ✅ CI test succeeded

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