fix FlashMLA cudagraph config (#4691)

Co-authored-by: yinfan98 <1106310035@qq.com>

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

@@ -92,7 +92,7 @@ class FlashMLABackend(FlashInferMLAAttnBackend):
         if forward_batch.forward_mode.is_decode_or_idle():
             if spec_info is None:
                 max_seqlen_pad = triton.cdiv(
-                    forward_batch.seq_lens.max().item(), PAGE_SIZE
+                    forward_batch.decode_seq_lens_cpu.max().item(), PAGE_SIZE
                 )
                 block_kv_indices = torch.full(
                     (bs, max_seqlen_pad),
@@ -206,8 +206,10 @@ class FlashMLABackend(FlashInferMLAAttnBackend):
     ):
 
         if forward_mode.is_decode_or_idle():
+            assert seq_lens_cpu is not None
             seq_lens = seq_lens[:bs]
-            max_seqlen_pad = triton.cdiv(seq_lens.max().item(), PAGE_SIZE)
+            seq_lens_cpu = seq_lens_cpu[:bs]
+            max_seqlen_pad = triton.cdiv(seq_lens_cpu.max().item(), PAGE_SIZE)
             create_flashmla_kv_indices_triton[(bs,)](
                 self.req_to_token,
                 req_pool_indices[:bs],