[DOC] Document common computer vision patterns

[ymrohit]

Fixes #8831

Summary

• Add an Advanced docs page for working with computer vision models in ExecuTorch.
• Cover preprocessing placement, resize/crop behavior, dtype conversion, normalization, and NCHW versus NHWC tensor layouts.
• Add Android Bitmap and iOS UIImage image-to-tensor examples, plus quick guidance for classifier, segmentation, detection, and instance segmentation outputs.
• Link the page from the Advanced section, Advanced pathway, and Getting Started MobileNet flow.

Test plan

• git diff --check origin/main..HEAD
• python -m compileall -q docs/source/conf.py docs/source/custom_directives.py
• PyTorch export smoke test for the preprocessing wrapper shown in the new page
• cd docs && MPLCONFIGDIR=/tmp/matplotlib-executorch-8831 PYTHONPATH=../pip-out/lib.linux-x86_64-cpython-312 python -m sphinx -D plot_gallery=0 -b html --keep-going source _build/html source/working-with-cv-models.md source/advanced-topics-section.md source/pathway-advanced.md source/getting-started.md
• Verified the rendered changed pages contain no local WARNING, ERROR, CRITICAL, system-message, or problematic artifacts

cc @mergennachin @AlannaBurke @byjlw

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19518

• 📄 Preview Python docs built from this PR

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❌ 1 New Failure, 5 Unrelated Failures, 1 Unclassified Failure

As of commit 57a6fb7 with merge base e25369b ():

NEW FAILURE - The following job has failed:

• pull / unittest-editable / macos / macos-job (gh)
  exir/tests/test_joint_graph.py::TestJointGraph::test_joint_graph

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:

• periodic (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)

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

• pull / unittest-editable / windows / windows-job (gh) (similar failure)
  exir/tests/test_joint_graph.py::TestJointGraph::test_joint_graph

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

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

• pull / unittest / linux / linux-job (gh) (trunk failure)
  exir/tests/test_memory_planning.py::TestMisc::test_multiple_pools_1
• pull / unittest / macos / macos-job (gh) (trunk failure)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv2_model
• pull / unittest / windows / windows-job (gh) (trunk failure)
  exir/tests/test_joint_graph.py::TestJointGraph::test_joint_graph
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)
  exir/tests/test_joint_graph.py::TestJointGraph::test_joint_graph

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

[ymrohit]

@pytorchbot label "release notes: none"

[GregoryComer]

Overall, this looks really good. I tagged a couple of platform-specific experts on the Android and iOS details. Thanks!

[GregoryComer]

@psiddh @kirklandsign Could you take a look at the Android-specific code here? Thanks!

[GregoryComer]

@shoumikhin @metascroy Could you take a look at the iOS code here? Thanks!