[CI/Build] tests(v1): feed Triton attention the (num_blocks, 2, …) KV cache layout in backend-correctness tests (#26663)

Signed-off-by: Huamin Li <3ericli@gmail.com>
Co-authored-by: Ye (Charlotte) Qi <yeq@meta.com>

Author: hl475

tests/v1/attention/test_attention_backends.py

@@ -423,13 +423,14 @@ def _test_backend_correctness(
     for backend_name in backend_to_test:
         # FlashAttentionm + FlexAttention:
         #   [2, num_blocks, block_size, num_kv_heads, head_size]
-        # FlashInfer:
+        # FlashInfer + Triton:
         #   [num_blocks, 2, block_size, num_kv_heads, head_size]
         # Select the appropriate KV cache format for each backend
         kv_cache_for_backend = kv_cache
-        if backend_name == _Backend.FLASHINFER:
+        if backend_name in (_Backend.FLASHINFER, _Backend.TRITON_ATTN):
             kv_cache_for_backend = kv_cache.transpose(0, 1)
 
+        if backend_name == _Backend.FLASHINFER:
             # For FlashInfer default to HND layout and
             kv_cache_for_backend = (
                 kv_cache_for_backend.transpose(2, 3).contiguous().transpose(2, 3)