Arm backend: Merge decompose/convert meandim pass

[AdrianLundell]

This change has multiple benifits:

• Cleaner arm_pass_manager
• Use more efficient avgpool2d decompostion in more cases
• Fixes a bug decomposing to avgpool for rank != 4

Note that symmetric_io_quantization is required for the unittests resulting in only a avgpool op becuase of the way avgpool is annotated.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

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• [PREEMPTIVE] Removal of ephemeral variants on scale-config.yml

❌ 2 New Failures

As of commit a7a25c2 with merge base dcd25eb ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / linux / linux-job (gh)
  extension/llm/modules/test/test_position_embeddings.py::TiledTokenPositionalEmbeddingTest::test_tiled_token_positional_embedding_et
• trunk / test-llama-runner-mac (fp32, mps) / macos-job (gh)
  The process '/usr/bin/git' failed with exit code 128

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[zingo]

@AdrianLundell this seem to have got a merge conflict :( and probably need a rebase.