[CUDA] Make sanity check optional for dgl.create_block.

[mfbalin]

Description

The hetero sampling case has too many GPU synchronizations, we need to hunt them down one by one if we want to get good performance.

I would appreciate any help in this direction as some of these are inside DGL itself, not GraphBolt.

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: bb989f9

Build ID: 1

Status: ⚪️ CI test cancelled due to overrun.

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[dgl-bot]

Commit ID: 7bd258f

Build ID: 2

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

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Commit ID: c4f323f

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Status: ⚪️ CI test cancelled due to overrun.

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Commit ID: bf587d4

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Status: ⚪️ CI test cancelled due to overrun.

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Commit ID: 47e46c2

Build ID: 5

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

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[dgl-bot]

Commit ID: 7a8f134

Build ID: 6

Status: ✅ CI test succeeded.

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