[Bugfix] Replace c10::optional with std::optional in topk kernel

[FloatingVertex]

Purpose

Replaced the usage of c10::optional with std::optional as c10::optional is deprecated.

The project has migrated away from it in prior PRs (#11730, #17819, #25602) and header for this function is already using std::optional.

Test Plan

CI

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[yeqcharlotte]

could you fix DCO?

[gemini-code-assist]

Code Review

This pull request replaces the deprecated c10::optional with std::optional in the large_context_topk kernel function. The change is correct and aligns with the ongoing effort to migrate away from deprecated c10 utilities. The updated function signature is consistent with its usage and the function it calls (get_params), which already uses std::optional. This is a good code health improvement.