Fix handling constant inputs when delegating (#3031)

Summary: Pull Request resolved: #3031

Differential Revision: D56089279

Author: angelayi

exir/lowered_backend_module.py

@@ -454,17 +454,28 @@ def _get_new_signature(  # noqa: C901
     new_state_dict = {}
     new_constants = {}
 
-    input_tensor_node_to_sig = {
-        input_spec.arg.name: input_spec
-        for input_spec in old_signature.input_specs
-        if isinstance(input_spec.arg, TensorArgument)
-    }
+    if tag is None:
+        # This is only the case where we're reconstructing the graph signature
+        # for the toplevel graph
+        placeholder_nodes = [
+            node.name
+            for node in original_program.graph.nodes
+            if node.op == "placeholder"
+        ]
+        assert len(placeholder_nodes) == len(old_signature.input_specs)
+        input_node_to_sig = dict(zip(placeholder_nodes, old_signature.input_specs))
+    else:
+        input_node_to_sig = {
+            input_spec.arg.name: input_spec
+            for input_spec in old_signature.input_specs
+            if isinstance(input_spec.arg, TensorArgument)
+        }
 
     for node in gm.graph.nodes:
         is_tagged = tag is None or node.meta.get("delegation_tag", None) == tag
         if node.op == "placeholder":
 
-            if node.name not in input_tensor_node_to_sig:
+            if node.name not in input_node_to_sig:
                 assert tag is not None
                 input_specs.append(
                     InputSpec(
@@ -475,7 +486,7 @@ def _get_new_signature(  # noqa: C901
                 )
                 continue
 
-            orig_input_spec = input_tensor_node_to_sig[node.name]
+            orig_input_spec = input_node_to_sig[node.name]
 
             if not isinstance(orig_input_spec.arg, TensorArgument):
                 input_specs.append(orig_input_spec)