[Core][CUDA Graph] add output buffer for cudagraph (#5074)

[Core][CUDA Graph] add output buffer for cudagraph to reduce memory footprint (#5074)

vllm/worker/model_runner.py

+import gc
 import time
 import warnings
 from collections import defaultdict
@@ -894,6 +895,10 @@ class ModelRunner:
         seq_lens = torch.ones(max_batch_size, dtype=torch.int32).cuda()
         block_tables = torch.from_numpy(self.graph_block_tables).cuda()
 
+        # Prepare buffer for outputs. These will be reused for all batch sizes.
+        # It will be filled after the first graph capture.
+        hidden_states: Optional[torch.Tensor] = None
+
         graph_batch_size = _get_graph_batch_size(
             self.scheduler_config.max_num_seqs)
         batch_size_capture_list = [
@@ -930,9 +935,11 @@ class ModelRunner:
                     self.set_active_loras(set(), lora_mapping)
 
                 graph_runner = CUDAGraphRunner(self.model)
-                graph_runner.capture(
+                hidden_states = graph_runner.capture(
                     input_tokens[:batch_size],
                     input_positions[:batch_size],
+                    hidden_states[:batch_size]
+                    if hidden_states is not None else None,
                     kv_caches,
                     attn_metadata,
                     memory_pool=self.graph_memory_pool,
@@ -969,12 +976,13 @@ class CUDAGraphRunner:
         self,
         input_ids: torch.Tensor,
         positions: torch.Tensor,
+        hidden_states: Optional[torch.Tensor],
         kv_caches: List[torch.Tensor],
         attn_metadata: AttentionMetadata,
         memory_pool: Optional[Tuple[int, int]],
         stream: torch.cuda.Stream,
         **kwargs,
-    ) -> None:
+    ) -> torch.Tensor:
         assert self._graph is None
         # Run the model a few times without capturing the graph.
         # This is to make sure that the captured graph does not include the
@@ -993,13 +1001,21 @@ class CUDAGraphRunner:
         # Capture the graph.
         self._graph = torch.cuda.CUDAGraph()
         with torch.cuda.graph(self._graph, pool=memory_pool, stream=stream):
-            hidden_states = self.model(
+            output_hidden_states = self.model(
                 input_ids,
                 positions,
                 kv_caches,
                 attn_metadata,
                 **kwargs,
             )
+            if hidden_states is not None:
+                hidden_states.copy_(output_hidden_states)
+            else:
+                hidden_states = output_hidden_states
+            del output_hidden_states
+            # make sure `output_hidden_states` is deleted
+            # in the graph's memory pool
+            gc.collect()
         torch.cuda.synchronize()
 
         # Save the input and output buffers.
@@ -1012,7 +1028,7 @@ class CUDAGraphRunner:
             "block_tables": attn_metadata.decode_metadata.block_tables,
         }
         self.output_buffers = {"hidden_states": hidden_states}
-        return
+        return hidden_states
 
     def forward(
         self,