[GraphBolt][CUDA] Refactor IndexSelectCSC and add output_size argument

[mfbalin]

Description

Refactoring the implementation so that if the indptr tensor is already sliced, then we don't need to slice it again and again when we call IndexSelectCSC. Also adding output_size argument so that synchronization can be eliminated if it is available.

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

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