Arm backend: Do not skip _fuse_duplicate_users pass

[mansnils]

Removes previous workaround of skipping a pass for VGF backend, as the actual root cause (GRPHCOMP-3140) has been resolved.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @Sebastian-Larsson @robell

[pytorch-bot]

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As of commit 48bb74c with merge base e281726 ():

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

• pull / unittest / macos / macos-job (gh) (trunk failure)
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• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv2_model
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• trunk / unittest-release / macos / macos-job (gh) (trunk failure)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv2_model
• trunk / unittest-release / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[Copilot]

Pull request overview

This PR removes a VGF-specific workaround that disabled the _fuse_duplicate_users pass in the Arm backend, now that the underlying issue (GRPHCOMP-3140) is resolved.

Changes:

• Removes the VgfCompileSpec override that disabled fuse_duplicate_users in the default pass pipeline config.
• Adds a unit test asserting that VGF’s default pass pipeline config enables fuse_duplicate_users.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
backends/arm/vgf/compile_spec.py	Stops overriding default pipeline config to disable fuse_duplicate_users, restoring base defaults.
backends/arm/test/misc/test_compile_spec.py	Adds coverage to ensure VGF defaults to FuseDuplicateUsersConfig.ENABLED.

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