[CK] Skip fp16 dropout d256 batch tests for compiler VGPR aliasing bug (#6342)

## Summary

- Skip fp16 FMHA forward dropout tests that use the d256 tile in batch
mode, gated on compiler version
- The AMDGPU compiler miscompiles these kernels due to VGPR aliasing of
Philox RNG parameters under high register pressure (383 VGPRs)
- bf16 dropout tests are unaffected and cover the same code paths

## Root Cause

The compiler aliases `ph_seed` and `ph_head_offset` (Philox RNG state
stored in VGPRs) with other live data during the softmax main loop. This
causes corrupted `buffer_store_byte` writes for dropout randval on wave
lanes 32-63, producing NaN in output and LSE tensors.

**Conditions:** fp16 + d256 tile + dropout + batch mode + `qr` pipeline
+ gfx90a

## Changes

- `include/ck_tile/core/config.hpp`: Add
`CK_TILE_WORKAROUND_ROCM_7_12_FP16_DROPOUT_MISCOMPILE` macro
- `test/ck_tile/fmha/test_fmha_fwd.cpp`: Version-gated `GTEST_SKIP` in
`TEST_P(Dropout, ...)`

## Test plan

- [x] ROCm 7.1.1 (clang 20): 168/168 fp16 dropout tests PASS (no skip
active)
- [x] ROCm 7.12 (clang 22): 132 PASS, 36 SKIPPED, 0 FAILED
- [x] bf16 dropout tests: 168/168 PASS (unaffected by this change)

include/ck_tile/core/config.hpp

@@ -209,6 +209,17 @@
 #endif
 #endif
 
+// workaround for AMDGPU compiler VGPR aliasing bug in dropout codegen (ROCm >= 7.12)
+// Philox RNG VGPR parameters get aliased under high register pressure (d256 tile).
+// fp16 is affected; bf16 is not (different type conversion codegen path).
+#ifndef CK_TILE_WORKAROUND_ROCM_7_12_FP16_DROPOUT_MISCOMPILE
+#if(HIP_VERSION_MAJOR == 7 && HIP_VERSION_MINOR >= 12) || (HIP_VERSION_MAJOR > 7)
+#define CK_TILE_WORKAROUND_ROCM_7_12_FP16_DROPOUT_MISCOMPILE 1
+#else
+#define CK_TILE_WORKAROUND_ROCM_7_12_FP16_DROPOUT_MISCOMPILE 0
+#endif
+#endif
+
 #ifndef CK_TILE_DEBUG_LOG
 #define CK_TILE_DEBUG_LOG 0
 #endif

test/ck_tile/fmha/test_fmha_fwd.cpp

@@ -601,6 +601,14 @@ TEST_P(Dropout, DataTypeConfig)
     auto [drop_seed, drop_offset, drop_prefs]                     = drop_seed_offset_prefs;
     auto [batch, nhead, nhead_k, seqlen_q, seqlen_k, mask_str]    = dims_mask;
 
+#if CK_TILE_WORKAROUND_ROCM_7_12_FP16_DROPOUT_MISCOMPILE
+    if constexpr(std::is_same_v<DataTypeConfig, FmhaFwdFp16>)
+    {
+        if(hdim_q > 128 && mode == mode_enum::batch)
+            GTEST_SKIP() << "Skipped: fp16 dropout d256 batch — compiler bug (ROCm >= 7.12)";
+    }
+#endif
+
     auto result = fmha_fwd_run<DataTypeConfig>(mode,
                                                batch,
                                                nhead,