immutable accessors in graph signature (#4433)

Summary:
Pull Request resolved: #4433

splitting ET part of D60253955

Reviewed By: guangy10, zhxchen17

Differential Revision: D60295940

fbshipit-source-id: 4ad9a661a50db9b9e9bccbc13b232416d7264a49

exir/passes/constant_prop_pass.py

@@ -212,11 +212,11 @@ def erase_constant_node(
 ) -> None:
     # Remove corresponding tensor from param/constants dict.
     signature = exported_program.graph_signature
-    if name := signature.inputs_to_parameters.pop(node.name, None):
+    if name := signature.inputs_to_parameters.get(node.name, None):
         exported_program.state_dict.pop(name, None)
-    elif name := signature.inputs_to_lifted_tensor_constants.pop(node.name, None):
+    elif name := signature.inputs_to_lifted_tensor_constants.get(node.name, None):
         exported_program.constants.pop(name, None)
-    elif name := signature.inputs_to_buffers.pop(node.name, None):
+    elif name := signature.inputs_to_buffers.get(node.name, None):
         exported_program.constants.pop(name, None)
         exported_program.state_dict.pop(name, None)
 

exir/program/_program.py

@@ -225,7 +225,7 @@ def lift_constant_tensor_pass(ep):
         return ep
 
     graph_signature = ep.graph_signature
-    buffers = graph_signature.buffers
+    buffers = list(graph_signature.buffers)
 
     fake_mode = list(ep.graph.nodes)[0].meta["val"].fake_mode
     first_user_input = None