[CK_TILE] fmha: Add query padding support to backward pass

[Jeff-Huang]

Proposed changes

[CK_TILE] fmha: Add query padding support to backward pass

Introduces support for query sequence padding (q_padding) in the FMHA backward pass kernels.
- Passing seqlen_q_ptr to the backward kernels to distinguish logical from physical sequence lengths.
- Updating OGradDotO, ConvertQGrad, and DQDKDV kernels to respect logical lengths and handle zero-length sequences.
- Aligning LSE indexing in the forward kernel with the padded layout for consistency.
- Adding a new GTest suite (test_fmha_bwd_kernel_padding.cpp) with comprehensive tests for various padding scenarios, Including zero-length sequences in fwd/bwd group mode

Unify sequence length and padding handling
- Replaced seqstart_padded_*_ptr with a more robust system that uses
seqstart_*_ptr for physical sequence lengths and introduces
seqlen_*_ptr and cu_seqlen_*_ptr for logical (unpadded) lengths.
- Established a clear order of precedence for determining sequence
length: cumulative lengths (cu_seqlen_*_ptr) take priority,
followed by per-sequence lengths (seqlen_*_ptr), and finally
physical lengths derived from seqstart_*_ptr.
- Clarified the distinction between "group mode" and "batch mode" and
how sequence lengths are handled in each case.
- Renamed cu_seqlen_kv_ptr to cu_seqlen_k_ptr for consistency.
- Updated comments and documentation to reflect the new argument
structure and usage.

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

Discussion

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