[V1] Enable V1 Fp8 cache for FA3 in the oracle (#15191)

Signed-off-by: Lucas Wilkinson <lwilkinson@neuralmagic.com>
Signed-off-by: Lucas Wilkinson <lwilkins@redhat.com>

vllm/engine/arg_utils.py

@@ -1562,9 +1562,20 @@ class EngineArgs:
 
         # No Fp8 KV cache so far.
         if self.kv_cache_dtype != "auto":
-            _raise_or_fallback(feature_name="--kv-cache-dtype",
-                               recommend_to_remove=False)
-            return False
+            fp8_attention = self.kv_cache_dtype.startswith("fp8")
+            will_use_fa = (
+                current_platform.is_cuda()
+                and not envs.is_set("VLLM_ATTENTION_BACKEND")
+            ) or envs.VLLM_ATTENTION_BACKEND == "FLASH_ATTN_VLLM_V1"
+            supported = False
+            if fp8_attention and will_use_fa:
+                from vllm.vllm_flash_attn.fa_utils import (
+                    flash_attn_supports_fp8)
+                supported = flash_attn_supports_fp8()
+            if not supported:
+                _raise_or_fallback(feature_name="--kv-cache-dtype",
+                                   recommend_to_remove=False)
+                return False
 
         # No Prompt Adapter so far.
         if self.enable_prompt_adapter: