[CK_TILE] Restrict FMHA codegen to the kernel subset used by
FlashAttention (#6038)
## Motivation
Currently, the CK FlashAttention integration generates a broader FMHA
kernel set than the FlashAttention wrappers can actually dispatch, which
increases compile time without improving runtime coverage.
## Technical Details
The FlashAttention CK wrappers do not use all logits/LSE variants
emitted by the default FMHA codegen. The direct `fmha_fwd` path always
uses softcap-disabled, LSE-enabled kernels, and the `fmha_fwd_splitkv`
path only uses softcap-disabled kernels. This change trims codegen to
that subset and stops generating the unused logits/LSE variants.
This reduces the generated forward kernel set without changing
`fmha_fwd_appendkv` or `fmha_bwd`. The reduced kernel set was validated
by building and running the
[FlashAttention](https://github.com/Dao-AILab/flash-attention) CK
backend.
Across targets, the total generated FMHA kernel count is reduced by:
- `gfx942`: 29.3%
- `gfx1100`: 33.7%
- `gfx1201`: 31.3%
## Test Plan
<!-- Explain any relevant testing done to verify this PR. -->
pytest test/test_flash_attn_ck.py from
https://github.com/Dao-AILab/flash-attention
## Test Result
all tests passed
<!-- Briefly summarize test outcomes. -->
## Submission Checklist
- [ ] Look over the contributing guidelines at
https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.
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