Arm backend: Support per-channel in TOSA.RESCALE

[oscarandersson8218]

Adds support for per-channel rescaling in TOSA dialect RESCALE op.

cc @freddan80 @per @zingo @digantdesai

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15267

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❌ 8 New Failures, 2 Pending, 3 Unrelated Failures

As of commit 23d7b69 with merge base fdfeaa4 ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t e3482fac27e7d669266e3e7efdfa095f8b4df1e0101be2862064ae81693cd830 /exec failed with exit code 1
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t 3319b6fc56901ddfbf7b358e6c22189f975f3cbed8ded48ddf03903fdd812149 /exec failed with exit code 1
• Test CUDA Builds / export-gemma3-cuda-non-quantized / linux-job (gh)
  RuntimeError: Command docker exec -t 0689163e45a75ff36e0a2dfc1d3906e00ee547500c994a7bc1b83df91ca001c4 /exec failed with exit code 2
• Test CUDA Builds / export-gemma3-cuda-quantized-int4-tile-packed / linux-job (gh)
  RuntimeError: Command docker exec -t 6dadc335af4871109b8b9ba8335c323a1e25e885a7667497802f07dc6726a1bd /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-non-quantized / linux-job (gh)
  RuntimeError: Command docker exec -t 9b1d48561da6b6ac54b266242dd40d7332b909289ea61e2759adeb481d74191b /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-quantized-int4-tile-packed / linux-job (gh)
  RuntimeError: Command docker exec -t c3da80e7a4de518339fd9c6b454bc235f768cd5414432a242ea05be80ff1f6b5 /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-quantized-int4-weight-only / linux-job (gh)
  RuntimeError: Command docker exec -t 1fc1ae4d836cd996fd6c44dc1538340c3ec4b5b9d9ea7493baecb6d5123ee745 /exec failed with exit code 2
• Test Metal Backend / export-voxtral-metal-artifact / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 2

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / linux / linux-job (gh) (trunk failure)
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)
• trunk / unittest-release / linux / linux-job (gh) (trunk failure)

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[Sebastian-Larsson]

Unrelated CI failures