[ROCM][Bugfix] Use correct stride in cp_mha_gather_cache_kernel for hybrid model (#37228) (#37228)

Signed-off-by: jennyyyyzhen <yzhen@hmc.edu>
Co-authored-by: yZhen <yZhen@fb.com>

vllm/v1/attention/backends/rocm_aiter_fa.py

@@ -59,6 +59,12 @@ if current_platform.is_rocm():
         head_size,
         x,
         max_block_num,
+        k_cache_stride0,
+        k_cache_stride1,
+        k_cache_stride2,
+        v_cache_stride0,
+        v_cache_stride1,
+        v_cache_stride2,
         DEQUANT: tl.constexpr,
         PAGE_SIZE: tl.constexpr,
         CACHE_FORMAT: tl.constexpr,
@@ -90,15 +96,15 @@ if current_platform.is_rocm():
             # V: [num_blocks, page_size, num_head, head_dim]
             key_cache_ptr_offset = (
                 key_cache_ptr
-                + block_id * num_heads * head_size * PAGE_SIZE
-                + slot_id * num_heads * head_size
-                + head_id * head_size
+                + block_id * k_cache_stride0
+                + slot_id * k_cache_stride1
+                + head_id * k_cache_stride2
             )
             value_cache_ptr_offset = (
                 value_cache_ptr
-                + block_id * num_heads * head_size * PAGE_SIZE
-                + slot_id * num_heads * head_size
-                + head_id * head_size
+                + block_id * v_cache_stride0
+                + slot_id * v_cache_stride1
+                + head_id * v_cache_stride2
             )
             k_reg = tl.load(key_cache_ptr_offset + col_offsets)
             v_reg = tl.load(value_cache_ptr_offset + col_offsets)
@@ -171,6 +177,11 @@ if current_platform.is_rocm():
         page_size = key_cache.shape[1]
         num_heads = key_cache.shape[2]
 
+        # Pass actual tensor strides so the kernel works correctly
+        # even when the cache is non-contiguous (e.g, for hybrid model)
+        k_strides = key_cache.stride()
+        v_strides = value_cache.stride()
+
         grid = lambda meta: (total_tokens, num_heads)
         cp_mha_gather_cache_kernel[grid](
             key_cache,
@@ -187,6 +198,12 @@ if current_platform.is_rocm():
             head_dim,
             x,
             block_tables.size(1),
+            k_strides[0],
+            k_strides[1],
+            k_strides[2],
+            v_strides[0],
+            v_strides[1],
+            v_strides[2],
             DEQUANT=dequant,
             PAGE_SIZE=page_size,
             CACHE_FORMAT=kv_cache_layout,
@@ -1337,6 +1354,8 @@ class AiterFlashAttentionImpl(AttentionImpl):
             assert k_scale is not None and v_scale is not None, (
                 "k_scale and v_scale are required for shuffled update"
             )
+            # TODO: Add correct KV cache handling for hybrid model. KV cache
+            # may not be contiguous if mamba state exists.
             reshape_and_cache_shuffle_triton(
                 key,
                 value,