Arm backend: Update weight observer for QAT

[zingo]

Arm backend: Update weight observer for QAT

Updates weight observer for QAT+per-channel. Also adds a simple test to
test QAT flow.

Signed-off-by: Oscar Andersson <[email protected]>

cc @digantdesai @freddan80 @per @oscarandersson8218

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14115

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❌ 3 New Failures, 3 Cancelled Jobs, 37 Pending, 2 Unrelated Failures

As of commit 389ba62 with merge base 0cac45b ():

NEW FAILURES - The following jobs have failed:

• pull / test-eval_llama-wikitext-linux / linux-job (gh)
  RuntimeError: Command docker exec -t d2b2844aedaa683e75d860c04abcca0847dcd71770e460e9d233f5606e48188b /exec failed with exit code 1
• trunk / test-torchao-huggingface-checkpoints (phi_4_mini) / linux-job (gh)
  RuntimeError: Command docker exec -t 8814fb546b7b216c1b84aa946eaa13945d2fc3515e5dbba20eb447af60494918 /exec failed with exit code 126
• trunk / test-torchao-huggingface-checkpoints (qwen3_4b) / linux-job (gh)
  RuntimeError: Command docker exec -t afb6e4173789d8ad52c620c32bd7885bf0226e0d51780f2884f73be5163b261a /exec failed with exit code 126

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• pull / test-binary-size-linux-gcc / linux-job (gh)
• pull / test-openvino-linux / linux-job (gh)
  ##[error]The operation was canceled.
• pull / test-setup-linux-gcc / linux-job (gh)
  ##[error]The operation was canceled.

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

• Apple / test-demo-ios / mobile-job (ios) (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following job failed but was present on the merge base:

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

• pull / test-moshi-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[zingo]

@digantdesai this is a fix on a commet you made in #13511

[facebook-github-bot]

@digantdesai has imported this pull request. If you are a Meta employee, you can view this in D82259448.

[digantdesai]

Internal CI looks green. We can merge it once GH CI is green.