[Backend Tester] Add subtest index field

[GregoryComer]

Some test functions run multiple test cases. Add a subtest index field to disambiguate these in the report.

[GregoryComer]

Stack from ghstack (oldest at bottom):

• [Backend Tester] Seed based on test name #13313
• [Backend Tester] Reduce log verbosity / spam #13312
• -> [Backend Tester] Add subtest index field #13311
• Link Vulkan backend with pybinding lib when built #13309
• [Backend Tester] Write report progressively #13308
• [Backend Tester] Clean up report output #13306
• [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] Add Qualcomm tester and register flow #12739

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13311

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 4 New Failures

As of commit 1c136d7 with merge base 3a02146 ():

NEW FAILURES - The following jobs have failed:

• Build documentation / build (buck2) / Build doc (gh)
  At least one of the pre-conditions you specified did not hold
• Lint / lintrunner / linux-job (gh)
  >>> Lint for backends/test/suite/models/__init__.py:
• 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 db84ac505b2188786f1db5cbf2444a3d28cbc9819f61d9dbc0e215c85f2f5133 /exec failed with exit code 1

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