[GraphBolt] Modify ItemSet/Dict requiring items to be tensors

[Skeleton003]

Description

From #7371 and #7382 . I tried to modify ItemSet/Dict directly and find it work well.

This PR intends to:

1. Limit the type of items, from Iterable to torch.Tensor.
2. Enable ItemSet/Dict to be indexed by iterbale of integers, which is required by the later update of ItemSampler that suppoorts 'real' stochastic sharding. A draft code can be found at https://github.com/dmlc/dgl/pull/7371/files#diff-3edfa80a59c2dbce85a71e629bec7b14faaf9ffdeefe3b0767c398032eb71d79R939-R941 .

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
• I've leverage the tools to beautify the python and c++ code.
• The PR is complete and small, read the Google eng practice (CL equals to PR) to understand more about small PR. In DGL, we consider PRs with less than 200 lines of core code change are small (example, test and documentation could be exempted).
• All changes have test coverage
• Code is well-documented
• To the best of my knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change
• Related issue is referred in this PR
• If the PR is for a new model/paper, I've updated the example index here.

Changes

[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: 6e784b649c6b0cd013db7d335dd4e59155711440

Build ID: 1

Status: ❌ CI test failed in Stage [Lint Check].

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: dbc650b439376c8fc24abc5a1dc5a87f65a245fe

Build ID: 2

Status: ⚪️ CI test cancelled due to overrun.

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: be6e77e739f045dd8d1b22bf4537754d26639f43

Build ID: 3

Status: ✅ CI test succeeded.

Report path: link

Full logs path: link

[Skeleton003]

I'll open another PR to rename ItemSetDict to HeteroItemSet since there would be lots of changes. For now let's merge this first @frozenbugs @Rhett-Ying .

[dgl-bot]

Commit ID: dca9188

Build ID: 4

Status: ✅ CI test succeeded.

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: 8249ed4

Build ID: 5

Status: ✅ CI test succeeded.

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: d8ae487

Build ID: 6

Status: ✅ CI test succeeded.

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: b124d612d05dddbd8e8c419333336d93fdd2b9cf

Build ID: 7

Status: ✅ CI test succeeded.

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: 6e4e3bc49c1760d2fdf0d40ab5dc47cf81ca33d6

Build ID: 8

Status: ⚪️ CI test cancelled due to overrun.

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: 2645e5a09c44b2826366466e950fbfb1a7e48965

Build ID: 9

Status: ✅ CI test succeeded.

Report path: link

Full logs path: link