[export] fix lifted constants order for 0-input graphs

[pianpwk]

Summary:
With empty graphs, the graph.inserting_before(first_user_input = None) call turns into a graph.inserting_after(root) call, inverting the order of constant input nodes being inserted.

This fixes the issue by initializing to the first node in the graph (still valid if not a user input - only used for insertion).

Test Plan: test_export

Differential Revision: D63403514

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✅ You can merge normally! (4 Unrelated Failures)

As of commit b976510 with merge base b5c2a65 ():

BROKEN TRUNK - The following jobs failed but were present on the merge base:

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

• pull / linux-focal-cuda12.1-py3.10-gcc9-sm86 / test (default, 1, 5, linux.g5.4xlarge.nvidia.gpu) (gh) (trunk failure)
  inductor/test_flex_attention.py::TestFlexAttention::test_lse_masked_output_backend_flex_decode
• pull / linux-focal-cuda12.1-py3.10-gcc9-sm86 / test (default, 4, 5, linux.g5.4xlarge.nvidia.gpu) (gh) (trunk failure)
  inductor/test_flex_decoding.py::TestFlexDecoding::test_non_sparse_mulitple_block_size
• trunk / linux-focal-cuda12.4-py3.10-gcc9-sm86 / test (default, 1, 5, lf.linux.g5.4xlarge.nvidia.gpu) (gh) (trunk failure)
  inductor/test_flex_attention.py::TestFlexAttention::test_lse_masked_output_backend_flex_decode
• trunk / linux-focal-cuda12.4-py3.10-gcc9-sm86 / test (default, 4, 5, lf.linux.g5.4xlarge.nvidia.gpu) (gh) (trunk failure)
  inductor/test_flex_decoding.py::TestFlexDecoding::test_non_sparse_mulitple_block_size

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This pull request was exported from Phabricator. Differential Revision: D63403514

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D63403514

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D63403514

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D63403514

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D63403514

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D63403514

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@pytorchbot merge -f 'Landed internally'

(Initiating merge automatically since Phabricator Diff has merged, using force because this PR might not pass merge_rules.json but landed internally)

[pytorchmergebot]

Merge started

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