Restore VGF skip guards and preload_deps shape

[psiddh]

Summary:
Restores two pieces of test plumbing in backends/arm/test/ that were
inadvertently removed from pytorch/executorch on GitHub and are still
required for stable CI:

1. _VGF_ENABLED skip-guards in test_rewrite_conv_pass.py — without
   them, three VGF tests crash (rather than skip) on environments where
   LAVAPIPE_LIB_PATH is unset.
2. preload_deps shape in targets.bzl — the prior refactor silently
   dropped //executorch/kernels/quantized:custom_ops_generated_lib from
   every non-VGF arm test whenever runtime.is_oss or not _ENABLE_VGF.

The guards are no-ops on environments that have lavapipe configured, so
this is a strict safety improvement for OSS CI and a divergence fix for
fbsource. This supersedes the long-stuck fix-up diff D100742931 (which
will be abandoned).

Differential Revision: D104267179

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

[pytorch-bot]

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⏳ 45 Pending, 1 Unrelated Failure

As of commit f32e20f with merge base 1643611 ():

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

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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[meta-codesync]

@psiddh has exported this pull request. If you are a Meta employee, you can view the originating Diff in D104267179.

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[Copilot]

Pull request overview

Restores Arm backend test “plumbing” needed to keep CI stable when VGF/Vulkan emulation isn’t configured, and to ensure quantized custom ops registration is consistently preloaded for Arm tests.

Changes:

• Reintroduces pytest.skipif guards for VGF-specific conv rewrite tests when LAVAPIPE_LIB_PATH is not set, preventing crashes in non-VGF environments.
• Restores preload_deps composition so //executorch/kernels/quantized:custom_ops_generated_lib is always preloaded, while VGF-specific deps remain conditional.

Reviewed changes

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

File	Description
backends/arm/test/targets.bzl	Ensures custom_ops_generated_lib is always present in preload_deps, and conditionally appends VGF/Vulkan preload deps only when applicable.
backends/arm/test/passes/test_rewrite_conv_pass.py	Adds environment-based VGF enablement detection and skip guards for three VGF-only tests to avoid crashing when lavapipe isn’t configured.

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[psiddh]

unrelated failures