[Fix] Fix a bug for flashmla to run R1 model (#5875)

Co-authored-by: pengcuo <dgpengcuo@gmail.com>

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

@@ -241,6 +241,9 @@ class FlashMLABackend(FlashInferMLAAttnBackend):
                 seq_lens_cpu,
             )
 
+    def get_cuda_graph_seq_len_fill_value(self):
+        return 1024
+
     def forward_decode(
         self,
         q: torch.Tensor,