bwd v3 remove ck dependency

[JaxChen29]

Motivation

Remove the Composable Kernel (CK) dependency from the FMHA backward pass library when building with ONLY_FAV3=1. The V3 ASM kernels are self-contained .co binaries that don't need CK at compile time, but the build previously required CK headers (fmha_bwd.hpp, ck_tile/core.hpp, mask.hpp, etc.) even when only the ASM path was used. This decouples the V3 backward build from CK, enabling faster builds and removing the hard dependency on the CK submodule for ASM-only configurations.

Technical Details

New file: csrc/include/ck_tile_shim.h Minimal shim providing ck_tile:: types (stream_config, index_t, long_index_t, log2e_v, get_warp_size, launch_kernel) for compilation without CK.

csrc/include/aiter_hip_common.h Conditional include: #if ONLY_FAV3 includes ck_tile_shim.h, otherwise includes ck_tile/core.hpp. Safe for all other modules since #if ONLY_FAV3 evaluates to 0 when the macro is undefined.

csrc/include/mha_bwd.h Guarded #include "fmha_bwd.hpp" with #if !ONLY_FAV3 to skip the heavy CK FMHA headers when building V3-only.

csrc/cpp_itfs/mha_bwd.cu

Defined local mask_enum and compute_mask_coordinates() under #if ONLY_FAV3 to replace CK's mask_enum (from mask.hpp) and ck_tile::make_generic_attention_mask_coordinates_from_lr_window().
The SWA mask coordinate computation block is dual-pathed: #if ONLY_FAV3 uses the local function, #else uses the original CK function.
The CK fallback path in mha_bwd() (fmha_bwd(traits, ck_args, s)) remains guarded by the existing #if ONLY_FAV3 / #else block -- no change needed there.
aiter/jit/optCompilerConfig.json For module_fmha_v3_bwd and module_fmha_v3_varlen_bwd:

Removed extra_include: [CK_DIR/example/ck_tile/01_fmha]
Removed flags_extra_hip: ['-DCK_TILE_FMHA_FWD_FAST_EXP2=1']
Kept flags_extra_cc: ['-DONLY_FAV3=1'] and blob_gen_cmd (ASM codegen, not CK-related).
op_tests/cpp/mha/build_mha.sh Updated bwd_v3 benchmark linking: the benchmark binary (bwd.exe) still uses CK include paths for its reference computation (CK is only removed from the library, not the test harness).

Test Plan

Test Result

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.