[CK_TILE] Fix Int32 Overflow in Deterministic FMHA BWD

[DDEle]

Proposed changes

This PR fixes an integer overflow issue in the deterministic FMHA backward implementation by changing batch/nhead stride variables from index_t (32-bit) to long_index_t (64-bit) for the dq_acc. See also ROCm/aiter#1873.

Checklist

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• I have added tests relevant to the introduced functionality, and the unit tests are passing locally
• I have added the test to REGRESSION_TESTS list defined at the top of CMakeLists.txt in tests/CMakeLists.txt, IF the test takes more than 30 seconds to run.
• I have added inline documentation which enables the maintainers with understanding the motivation
• I have removed the stale documentation which is no longer relevant after this pull request
• (If this change is user-facing) I have added release notes which provide the end users with a brief summary of the improvement from this pull request
• I have run clang-format on all changed files
• Any dependent changes have been merged

[Copilot]

Pull request overview

This PR fixes an integer overflow issue in the deterministic FMHA (Fused Multi-Head Attention) backward pass implementation by changing stride variables from index_t (32-bit) to long_index_t (64-bit) for the dq_acc accumulator tensor.

Changes:

• Changed nhead_stride_dq_acc and batch_stride_dq_acc from ck_tile::index_t to ck_tile::long_index_t across kernel definitions and argument structures
• Restructured the dq_acc tensor layout to be [batch, nhead, nsplits, seqlen_q, hdim_q] instead of conditional layouts
• Updated stride calculations to use proper casting and correct dimension ordering

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
include/ck_tile/ops/fmha/kernel/fmha_bwd_kernel.hpp	Changed type of nhead_stride_dq_acc and batch_stride_dq_acc from index_t to long_index_t in kernel structs and function signatures
example/ck_tile/01_fmha/fmha_bwd_runner.hpp	Simplified dq_acc tensor layout, corrected stride calculations with proper casting, and reordered computation of stride variables
example/ck_tile/01_fmha/fmha_bwd.hpp	Updated argument structure to use long_index_t for nhead_stride_dq_acc and batch_stride_dq_acc

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