[algo] feat: add Focal-GRPO implementation

[alekseymalakhov11]

What does this PR do?

This PR adds the official implementation of F-GRPO. It introduces focal weighting into GRPO/DAPO/CISPO-style loss functions, improving pass@k performance while maintaining comparable pass@1 results.

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[gemini-code-assist]

Code Review

This pull request introduces a well-structured implementation of the F-GRPO algorithm. The changes are consistent across configuration files and the core algorithm logic. The new compute_f_grpo_outcome_advantage function is cleanly implemented, leveraging existing utilities for efficiency and includes robust parameter validation. I've identified a minor but important documentation issue regarding the paper reference, which I've commented on.

[alekseymalakhov11]

This also resolves #5256
fyi @Felix-Zhenghao

[alekseymalakhov11]

It looks like the other failing tests are unrelated to this PR and are also failing on the main branch.

I’d appreciate it if someone could take a look and review my PR
@PeterSH6 @tongyx361 @vermouth1992 @eric-haibin-lin