[NVIDIA] flashinfer TRTLLM attention prefill token limit (#25998)

Signed-off-by: jasonlizhengjian <jason.li@centml.ai>
Signed-off-by: jasonlizhengjian <jasonlizhengjian@gmail.com>

vllm/utils/flashinfer.py

@@ -283,11 +283,18 @@ def use_trtllm_attention(
 
     if force_use_trtllm is None:
         # Environment variable not set - use auto-detection
-        use_trtllm = (
-            num_tokens <= 256 and max_seq_len <= 131072 and kv_cache_dtype == "auto"
-        )
-        if use_trtllm:
-            logger.warning_once("Using TRTLLM attention (auto-detected).")
+        if is_prefill:
+            # Prefill auto-detection
+            use_trtllm = max_seq_len <= 131072 and kv_cache_dtype == "auto"
+            if use_trtllm:
+                logger.warning_once("Using TRTLLM prefill attention (auto-detected).")
+        else:
+            # Decode auto-detection
+            use_trtllm = (
+                num_tokens <= 256 and max_seq_len <= 131072 and kv_cache_dtype == "auto"
+            )
+            if use_trtllm:
+                logger.warning_once("Using TRTLLM decode attention (auto-detected).")
         return use_trtllm
 
     # Environment variable is set to 1 - respect it