[CK_TILE] Adjust kBlockSize of reduce example for better perf

[ClementLinCF]

Proposed changes

• Observed a 2x perf improvement with kBlockSize = 256
• Using 512 threads may lead to redundant computations

Checklist

Please put an x into the boxes that apply. You can also fill these out after creating the PR. If you're not sure, please don't hesitate to ask.

• [x ] I have added tests relevant to the introduced functionality, and the unit tests are passing locally
• I have added inline documentation which enables the maintainers with understanding the motivation
• I have removed the stale documentation which is no longer relevant after this pull request
• (If this change is user-facing) I have added release notes which provide the end users with a brief summary of the improvement from this pull request
• [ x] I have run clang-format on all changed files
• [x ] Any dependent changes have been merged

Discussion

If this is a relatively large or complex change, feel free to start a discussion by explaining why you chose the solution you did and what alternatives you considered

[carlushuang]

LGTM