Arm backend: Generalize RemovePermutesAroundElementwiseTosaOps

[AdrianLundell]

• Use is_param_node for finding constant placeholders
• Ensure ops are not modified by multiple subgraphs

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

[pytorch-bot]

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[Copilot]

Pull request overview

This PR generalizes the Arm TOSA pass that removes redundant permutes around elementwise ops by improving constant-placeholder detection and preventing invalid rewrites when multiple candidate subgraphs overlap.

Changes:

• Add a “stale subgraph” guard to skip applying rewrites when a previously-applied rewrite has already changed a candidate’s boundary edges.
• Update RemovePermutesAroundElementwiseTosaOps to use is_param_node(exported_program, node) for identifying constant placeholders, and thread exported_program through the Arm pass pipeline.
• Expand and update Arm backend tests to cover folded scalar constants and shared-boundary/stale-subgraph behavior; adjust transpose count expectations accordingly.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
backends/transforms/remove_permutes_around_elementwise_ops.py	Adds _subgraph_edges_are_current() and checks it before rewriting a candidate subgraph.
backends/arm/_passes/remove_permutes_around_elementwise_tosa_ops.py	Requires ExportedProgram, improves constant detection via is_param_node, and keeps TABLE constant inputs from being permuted.
backends/arm/_passes/arm_pass_manager.py	Wires exported_program into the RemovePermutesAroundElementwiseTosaOps pass construction.
backends/arm/test/passes/test_remove_permutes_around_elementwise_tosa_ops.py	Adds new unit tests for folded scalar constants and stale shared-boundary subgraphs; updates pass construction to provide an exported program.
backends/arm/test/misc/test_transpose_counts.py	Updates expected transpose count for a model affected by improved permute-removal behavior.

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