[FEA] Custom index type of mdspan.

[trivialfis]

The upstream version of mdspan from #437 uses size_t for indexing the memory buffer. While size_t is a good choice for many cases as it can prevent integer overflow errors, it also increases register pressure and might have performance overhead. We can opt to use other smaller indexing types by removing the conversion as done in trivialfis/mdspan@f18e694 .

[github-actions]

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[github-actions]

This issue has been labeled inactive-90d due to no recent activity in the past 90 days. Please close this issue if no further response or action is needed. Otherwise, please respond with a comment indicating any updates or changes to the original issue and/or confirm this issue still needs to be addressed.