[CI Failure] Fix fp8 kv cache on <SM90 (#25396)

Signed-off-by: mgoin <mgoin64@gmail.com>

vllm/platforms/cuda.py

@@ -286,6 +286,9 @@ class CudaPlatformBase(Platform):
             TREE_ATTN_V1 = "vllm.v1.attention.backends.tree_attn.TreeAttentionBackend"  # noqa: E501
             XFORMERS_V1 = "vllm.v1.attention.backends.xformers.XFormersAttentionBackend"  # noqa: E501
 
+            use_fp8_kv_cache = (kv_cache_dtype is not None
+                                and kv_cache_dtype.startswith("fp8"))
+
             if selected_backend == _Backend.FLASHINFER:
                 logger.info_once("Using FlashInfer backend on V1 engine.")
                 if cls.has_device_capability(100):
@@ -334,10 +337,11 @@ class CudaPlatformBase(Platform):
 
             # FlashAttention is the default for SM 8.0+ GPUs
             if cls.has_device_capability(80):
-                if has_sink and not cls.is_device_capability(90):
+                if (has_sink or
+                        use_fp8_kv_cache) and not cls.is_device_capability(90):
                     logger.info_once("Using Triton backend on V1 engine.")
                     return TRITON_ATTN_VLLM_V1
-                if is_default_backend_supported := is_attn_backend_supported(
+                elif is_default_backend_supported := is_attn_backend_supported(
                         FLASH_ATTN_V1, head_size, dtype,
                         allow_import_error=False):
                     logger.info_once("Using Flash Attention backend on "