[Backend Tester] Add portable test flow

[GregoryComer]

Add a portable tester flow to the backend tester. This is mainly intended to help validate the test suite and to get baseline numbers to compare against delegated results.

[GregoryComer]

Stack from ghstack (oldest at bottom):

• [Backend Tester] Clean up a few test issues #13258
• -> [Backend Tester] Add portable test flow #13250
• [Backend Tester] Report PTE size #13249
• [Backend Tester] Report delegation statistics #12846
• [Backend Tester] Report quantization and lowering times #12838
• [Backend Tester] Add tensor error statistic reporting #12809
• [Backend Tester] Add CSV report generation #12741
• [Backend Tester] [WIP] Add Qualcomm tester and register flow #12739

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13250

• 📄 Preview Python docs built from this PR

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❌ 4 New Failures, 1 Unrelated Failure

As of commit c23c3e9 with merge base 09d5255 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner / linux-job (gh)
  >>> Lint for backends/test/suite/flow.py:
• pull / test-moshi-linux / linux-job (gh)
  RuntimeError: Command docker exec -t f4b535519a2f54a63d608d657d24fed1fddd8ed4993df54940f780757f5ea8dd /exec failed with exit code 1
• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh)
  backends/arm/test/models/stable_diffusion/test_vae_AutoencoderKL.py::TestAutoencoderKL::test_AutoencoderKL_tosa_MI
• pull / unittest-arm-backend-with-no-fvp (test_pytest_ops) / linux-job (gh)
  RuntimeError: Command docker exec -t 3e4e8ca0ab65b1086425fd25ec2615dbf3e7e3426b58b095a57efa9424c7c42b /exec failed with exit code 1

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

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

• pull / test-openvino-linux / linux-job (gh) (trunk failure)
  AttributeError: '_OpNamespace' 'quantized_decomposed' object has no attribute 'convert_element_type'

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