[CK Tile] Add sink token gradient support in FMHA backward
pass (#5504)
## Motivation
Adds sink token support to the FMHA backward kernel (dot_do_o pipeline):
## Technical Details
- Extend BlockFmhaBwdOGradDotOPipelineProblem with LSEDataType
- Add sink_ptr/d_sink_ptr/lse_ptr/nhead to FmhaBwdOGradDotOCommonKargs
- Compute per-head sink gradient via atomic accumulation in the pipeline
- Update example runner with reference validation for sink gradient
## Test Plan
Add new test case
## Test Result
WIP
## Submission Checklist
- [ ] Look over the contributing guidelines at
https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.
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