support multi-step draft-model with cudagraph (#5886)

fastdeploy/spec_decode/mtp.py

@@ -708,7 +708,7 @@ class MTPProposer(Proposer):
         self.model_inputs["not_need_stop"][0] = True
         self.model_inputs["seq_lens_this_time"] = self.seq_lens_this_time_buffer
 
-    def _initialize_forward_meta(self, step_use_cudagraph: bool = False):
+    def _initialize_forward_meta(self, step_use_cudagraph: bool = False, is_dummy_run: bool = False, substep: int = 0):
         """
         Initialize forward meta and attention meta data
         """
@@ -744,7 +744,12 @@ class MTPProposer(Proposer):
         for attn_backend in self.attn_backends:
             attn_backend.init_attention_metadata(self.forward_meta)
 
-        self.forward_meta.step_use_cudagraph = step_use_cudagraph and self.draft_model_use_cudagraph
+        # Notes(liuzichang):
+        # 1. CUDA Graph capture sizes must be recorded in descending order (large → small).
+        # 2. In multi-step execution, only the first step should be captured.
+        self.forward_meta.step_use_cudagraph = (
+            step_use_cudagraph and self.draft_model_use_cudagraph and not (substep > 0 and is_dummy_run)
+        )
 
     def _initialize_forward_meta_xpu(self):
 
@@ -922,7 +927,9 @@ class MTPProposer(Proposer):
                 self.model_inputs["output_padding_offset"].copy_(output_padding_offset, False)
 
                 # Initialize forward meta data
-                self._initialize_forward_meta(step_use_cudagraph=step_use_cudagraph)
+                self._initialize_forward_meta(
+                    step_use_cudagraph=step_use_cudagraph, is_dummy_run=is_dummy_run, substep=substep
+                )
                 self.forward_meta.batch_id_per_token.copy_(batch_id_per_token, False)
 
                 # Padding inputs for cuda graph
@@ -947,9 +954,10 @@ class MTPProposer(Proposer):
                     top_p_normalized_logprobs=self.model_inputs["top_p_normalized_logprobs"],
                     share_inputs=self.model_inputs,
                 )
-
+                # Note(liuzichang):
+                # paddle.clone would raise error 700 in cudaGraph mode
                 if self.num_model_steps > 1:
-                    self.last_seq_lens_this_time = paddle.clone(self.model_inputs["seq_lens_this_time"])
+                    self.last_seq_lens_this_time.copy_(self.model_inputs["seq_lens_this_time"], False)
 
                 model_output = self.model(
                     ids_remove_padding=self.model_inputs["ids_remove_padding"],