Fix fake mode detection

[angelayi]

Differential Revision: D87577206

[pytorch-bot]

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❌ 1 New Failure, 2 Unrelated Failures

As of commit 8083ea7 with merge base b4d72f1 ():

NEW FAILURE - The following job has failed:

• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh)
  RuntimeError: Command docker exec -t f58e20174eb2b48b530c4fc33cef37bb5d9d2765dd8c36c69798191aae176d49 /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest-editable / linux / linux-job (gh) (similar failure)
  exir/backend/test/test_lowered_backend_module.py::TestBackendAPI::test_emit_nested_lowered_backend_module

BROKEN TRUNK - The following job failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / linux / linux-job (gh) (trunk failure)
  exir/backend/test/test_lowered_backend_module.py::TestBackendAPI::test_emit_nested_lowered_backend_module

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[Copilot]

Pull Request Overview

This PR fixes the fake mode detection in the lift_constant_tensor_pass function by using the more robust _detect_fake_mode_from_gm utility function instead of directly accessing the fake mode from the first node's metadata. This addresses a known issue where the first node may not always have the expected metadata structure, particularly when the first node is a scalar.

• Replaces brittle metadata access with a dedicated utility function
• Imports _detect_fake_mode_from_gm from torch._export.utils
• Adds a blank line for better code formatting

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