Arm backend: allow global register for ArmBackend pass insertions

[robell]

To make the use of custom ops easier (which rely on passes to handle rewrite
and prevent decomposition) - provide a simple mechanism to insert them into
the pass manager using existing functionality.

Also a small commit which fixes missing doc updates.

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

[pytorch-bot]

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• trunk / test-torchao-huggingface-checkpoints (phi_4_mini, linux.arm64.2xlarge, executorch-ubuntu-22.04-gc... / linux-job (gh)
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• trunk / test-torchao-huggingface-checkpoints (qwen3_4b, linux.arm64.2xlarge, executorch-ubuntu-22.04-gcc1... / linux-job (gh)
  RuntimeError: Command docker exec -t 34b7d0b93f48c69672214585e7cc6ad8bf1631122b0fef04d13aa6a4ed666f7c /exec failed with exit code 1

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[Copilot]

Pull request overview

Arm backend update to enable globally registering pass insertions into ArmPassManager pipelines (useful for custom ops that need rewrite/prevent decomposition), plus documentation updates to mention custom partition ops registration.

Changes:

• Add a module-level registry and helper functions to register pass insertions before/after a target pass type for all Arm pipelines.
• Apply globally registered pass insertions during Arm pipeline construction via _configure_pass_insertions.
• Update Arm VGF and Ethos-U partitioner docs to include register_custom_partition_op.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
docs/source/backends/arm-vgf/arm-vgf-partitioner.md	Documents register_custom_partition_op for the VGF partitioner API reference.
docs/source/backends/arm-ethos-u/arm-ethos-u-partitioner.md	Documents register_custom_partition_op for the Ethos-U partitioner API reference.
backends/arm/_passes/arm_pass_manager.py	Introduces global pass insertion registration and applies those insertions during pipeline setup.

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