[GraphBolt] Optimize hetero sampling on CPU

[RamonZhou]

Description

Moved from #7304.
The CPU part of #7223.

What has changed:

• Hetero sampling when fanouts.size() > 0

What hasn't changed:

• Temporal sampling (the effect is unsure).
• Hetero graph but fanouts.size() = 1 (this code change doesn't apply to this situation sufficiently. It needs to be implemented in another way).
• Homo sampling (shouldn't be changed).

The performance will be tested soon.

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

[mfbalin]

Why did we need to create another PR? You can just force push to the old PR branch if you wanted to get rid of some commits or anything like that?

[dgl-bot]

Commit ID: 9b45bc3e36667be2840bd5e4f11c242b1c559c84

Build ID: 1

Status: ❌ CI test failed in Stage [Torch CPU (Win64) Unit test].

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: d91a360

Build ID: 2

Status: ✅ CI test succeeded.

Report path: link

Full logs path: link

[mfbalin]

See #7368