Only allocate encoder metadata for encoder-decoder models (#16527)

python/sglang/srt/layers/attention/flashattention_backend.py

@@ -325,6 +325,7 @@ class FlashAttentionBackend(AttentionBackend):
             and model_runner.model_config.is_encoder_decoder
         ), "Sliding window and cross attention are not supported together"
 
+        self.is_encoder_decoder = model_runner.model_config.is_encoder_decoder
         self.forward_metadata: FlashAttentionMetadata = None
         # extra metadata for handling speculative decoding topk > 1, extended draft decode and verify
         self.forward_metadata_spec_decode_expand: FlashAttentionMetadata = None
@@ -1574,6 +1575,8 @@ class FlashAttentionBackend(AttentionBackend):
                     ),
                 }
 
+        # Only allocate encoder metadata for encoder-decoder models
+        if self.is_encoder_decoder:
             self.encoder_metadata = {
                 "encoder_page_table": torch.zeros(
                     max_bs,
@@ -1588,6 +1591,9 @@ class FlashAttentionBackend(AttentionBackend):
                     max_bs + 1, dtype=torch.int32, device=self.device
                 ),
             }
+        else:
+            # For decoder-only models, skip encoder_metadata allocation
+            self.encoder_metadata = {}
 
     def init_forward_metadata_capture_cuda_graph(
         self,