[Bugfix] Eliminate tuple inputs to submodules in graph partitioning (#28533)

Signed-off-by: Yanan Cao <gmagogsfm@gmail.com>

vllm/compilation/backends.py

@@ -4,6 +4,7 @@
 import ast
 import dataclasses
 import hashlib
+import operator
 import os
 import pprint
 import time
@@ -307,12 +308,24 @@ def split_graph(
 ) -> tuple[fx.GraphModule, list[SplitItem]]:
     # split graph by ops
     subgraph_id = 0
-    node_to_subgraph_id = {}
-    split_op_graphs = []
+    node_to_subgraph_id: dict[fx.Node, int] = {}
+    split_op_graphs: list[int] = []
     for node in graph.graph.nodes:
         if node.op in ("output", "placeholder"):
             continue
 
+        # Check if this is a getitem operation on a node from an earlier subgraph.
+        # If so, assign it to the same subgraph as its input to avoid passing entire
+        # tuple as input to submodules, which is against standalone_compile and
+        # AoTAutograd input requirement.
+        if node.op == "call_function" and node.target == operator.getitem:
+            # Assign this getitem to the same subgraph as its input
+            input_node = node.args[0]
+            if input_node.op != "placeholder":
+                assert input_node in node_to_subgraph_id
+                node_to_subgraph_id[node] = node_to_subgraph_id[input_node]
+                continue
+
         if should_split(node, splitting_ops):
             subgraph_id += 1
             node_to_subgraph_id[node] = subgraph_id