fix: Fix unbacked sym int not found issue

[peri044]

Description

Fixes #3588

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project (You can use the linters)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas and hacks
• I have made corresponding changes to the documentation
• I have added tests to verify my fix or my feature
• New and existing unit tests pass locally with my changes
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[zewenli98]

LGTM. just a quick question: add_var_to_val(k, v) would fail but set_unbacked_var_to_val(k, v) would work if k already exists right? The tests worked well in the previous. Any changes caused the issue?

[peri044]

LGTM. just a quick question: add_var_to_val(k, v) would fail but set_unbacked_var_to_val(k, v) would work if k already exists right? The tests worked well in the previous. Any changes caused the issue?

Yes, the tests failed with recent versions of Pytorch which went undetected. add_var_to_val wasn't adding the unbacked symint to var_to_val anymore throughout the torch.export tracing and it resulted in KeyError: u0. The recommended way by Meta to add unbacked symints is through set_unbacked_var_to_val and this ensures unbacked symints are available via node.shape_env.unbacked_var_to_val attribute.