Fix Piecewise CUDA Graph crash with -enable-mixed-chunk (#20441)

Co-authored-by: jianyingzhu <joeyzhu@nvidia.com>

python/sglang/srt/model_executor/piecewise_cuda_graph_runner.py

@@ -682,8 +682,20 @@ class PiecewiseCudaGraphRunner:
 
         next_token_logits_buffer = None
 
+        # Normalize MIXED→EXTEND so dynamo's guard (captured with EXTEND=1) doesn't fail on MIXED=3.
+        pcg_forward_mode = (
+            ForwardMode.EXTEND
+            if forward_batch.forward_mode == ForwardMode.MIXED
+            else forward_batch.forward_mode
+        )
+        pcg_global_forward_mode = (
+            ForwardMode.EXTEND
+            if forward_batch.global_forward_mode == ForwardMode.MIXED
+            else forward_batch.global_forward_mode
+        )
+
         static_forward_batch = ForwardBatch(
-            forward_mode=forward_batch.forward_mode,
+            forward_mode=pcg_forward_mode,
             batch_size=bs,
             input_ids=input_ids,
             input_embeds=input_embeds,
@@ -721,7 +733,7 @@ class PiecewiseCudaGraphRunner:
             spec_info=forward_batch.spec_info,
             capture_hidden_mode=forward_batch.capture_hidden_mode,
             num_token_non_padded=forward_batch.num_token_non_padded,
-            global_forward_mode=forward_batch.global_forward_mode,
+            global_forward_mode=pcg_global_forward_mode,
             lora_ids=forward_batch.lora_ids,
             sampling_info=forward_batch.sampling_info,
             mm_inputs=forward_batch.mm_inputs,