Add channels_last edge-op dialect (conv, avg_pool2d, permute_copy)

[rascani]

Summary

First vertical slice of the common memory-layout handling infra (RFC #19299): a backend-agnostic channels_last operator dialect whose ops interpret their activations as (N, H, W, C) contiguous with a fixed dim-order. Kernels wrap the existing aten ops with permutes, since efficiency is a non-goal at this layer; later passes (the #20094 conversion and #20098 fusion) make channels-last regions explicit and fuse the permutes away. Runtime C++ kernels, the conversion pass, and the remaining ops are deferred to follow-ups.

Part of #20093.

Test plan

pytest backends/transforms/test/test_channels_last_ops.py

[pytorch-bot]

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❌ 2 New Failures, 1 Unrelated Failure, 1 Unclassified Failure

As of commit 019d0f4 with merge base 92e6a4c ():

NEW FAILURES - The following jobs have failed:

• pull / android / run-emulator (gh)
  The process '/usr/bin/sh' failed with exit code 1
• pull / test-arm-backend-no-driver (test_pytest_models_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 3152a1fd4cf802b5cb07e8bf23019e3dd4b1f02ab3ea39a7035704aa61c62484 /exec failed with exit code 1

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• pull / test-arm-backend-public-api-backward-compatibility / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command docker exec -t df384b70f1a9fff08a14390e81a4b84c4afb86ab2b06e0cd95510b3b78068133 /exec failed with exit code 127

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

• pull / test-moshi-linux / linux-job (gh) (matched linux rule in flaky-rules.json)
  E: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/main/m/mesa/mesa-vdpau-drivers_23.2.1-1ubuntu3.1%7e22.04.3_amd64.deb 404 Not Found [IP: 104.20.28.246 80]

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