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[CK_TILE] Use gfx11 float buffer atomics in FMHA Bwd ## Motivation FlashAttention CK backward on gfx11 can hit out-of-bounds/tail writes in the dQ accumulator atomic-add path when sequence rows are padded at the tile level but not marked invalid in the DQDKDV main tensor view. With the generic global atomic fallback, an incorrectly-valid tail element can issue an actual pointer-based `atomicAdd`. With the buffer atomic path, the write is issued through a buffer resource with bounds information and follows the same backend already used by gfx9/gfx12. This fixes the gfx11 FMHA BWD failure without changing the gfx11 default for unrelated CK Tile kernels. ## Technical Details This PR enables the existing CK Tile AMD buffer float atomic-add path only for generated FMHA BWD gfx11 translation units. gfx11 normally uses the generic global atomic fallback for floating-point `buffer_view::atomic_add`. That fallback performs the atomic through a raw computed pointer and depends on the software validity predicate to avoid invalid elements. In FMHA BWD dQ accumulation, padded tail rows can reach this path, so using the buffer atomic backend is safer: it uses a buffer resource with base pointer, bounds information, and an element offset, matching the backend already used by gfx9/gfx12. Enabling `CK_TILE_USE_AMD_BUFFER_ATOMIC_ADD_FLOAT` globally for gfx11 is too broad and can break unrelated gfx11 CK builds such as GEMM. Instead, `config.hpp` now preserves an explicitly pre-defined `CK_TILE_USE_AMD_BUFFER_ATOMIC_ADD_FLOAT`, while keeping the existing default disabled for gfx11. ## Test Plan Validated the change with the FlashAttention CK full test suite with backward pass enabled on gfx11. pytest -q -s tests/test_flash_attn_ck.py ## Test Result FlashAttention CK gfx11 test result: 260680 passed, 152076 skipped ## Submission Checklist - [ ] Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests. Co-authored-by: Po Yen Chen <PoYen.Chen@amd.com>