Fix fake mode detection (#15931)

Summary:

it's not necessarily true that the 0th placeholder's meta["val"] contains a tensor. replaced this with _detect_fake_mode_from_gm which is safer

Reviewed By: cccclai

Differential Revision: D87577206

Author: angelayi

exir/program/_program.py

@@ -80,6 +80,7 @@
     get_aten_verifier,
 )
 from torch._export.passes import ReplaceViewOpsWithViewCopyOpsPass
+from torch._export.utils import _detect_fake_mode_from_gm
 from torch._export.verifier import Verifier
 from torch.export import ExportedProgram
 from torch.export._remove_auto_functionalized_pass import (
@@ -333,7 +334,8 @@ def lift_constant_tensor_pass(ep):
     graph_signature = ep.graph_signature
     buffers = list(graph_signature.buffers)
 
-    fake_mode = list(ep.graph.nodes)[0].meta["val"].fake_mode
+    fake_mode = _detect_fake_mode_from_gm(ep.graph_module)
+
     first_user_input = None
     lifted_constants = []
     for node in ep.graph.nodes: