Improve QNN backend AOT user experience

[abhinaykukkadapu]

Stack from ghstack (oldest at bottom):

• Improve QNN backend build-from-source user experience #17990
• -> Improve QNN backend AOT user experience #17989

• Read QNN_VERSION from qnn_config.sh instead of hardcoding in
  download_qnn_sdk.py, eliminating version duplication
• Add download progress bars for SDK (~2.4GB), libc++ (~575MB), and
  glibc (~10MB) downloads with percentage and MB counters
• Add extraction progress for SDK, libc++, and tar archives
• Add retry + resume for SDK download — on mid-stream failures
  (ChunkedEncodingError, ConnectionError), resumes from where it
  left off using HTTP Range headers instead of restarting
• Replace NullHandler logger with configured StreamHandler so users
  see status messages (previously all output was swallowed)
• Cache SDK at ~/.cache/executorch/qnn/sdk-/ instead of
  inside the pip package directory, surviving pip reinstalls
• Include version in cache directory name for automatic upgrade
  detection when qnn_config.sh version changes
• Set both QNN_SDK_ROOT and LD_LIBRARY_PATH after auto-download
• Print SDK version and path on first import (cached or fresh)
• Print user-provided QNN_SDK_ROOT path when set via env var
• Add visible warning before glibc re-exec with guidance
• Replace unhelpful "check logs above" error with actionable message
  including full SDK download URL and env var setup instructions

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17989

• 📄 Preview Python docs built from this PR

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

❌ 3 New Failures, 2 Unrelated Failures

As of commit 496f338 with merge base 258d503 ():

NEW FAILURES - The following jobs have failed:

• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model
• Test Metal Backend / export-model-metal-artifact (nvidia, parakeet-tdt, quantized-int4-metal) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• Test Metal Backend / export-model-metal-artifact (openai, whisper-small, non-quantized) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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

• Test Metal Backend / export-model-metal-artifact (mistralai, Voxtral-Mini-3B-2507, quantized-int4-metal) / macos-job (gh) (similar failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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

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

• Test Metal Backend / export-model-metal-artifact (mistralai, Voxtral-Mini-3B-2507, non-quantized) / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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

[github-actions]

This PR needs a release notes: label

If your change should be included in the release notes (i.e. would users of this library care about this change?), please use a label starting with release notes:. This helps us keep track and include your important work in the next release notes.

To add a label, you can comment to pytorchbot, for example
@pytorchbot label "release notes: none"

For more information, see
https://github.com/pytorch/pytorch/wiki/PyTorch-AutoLabel-Bot#why-categorize-for-release-notes-and-how-does-it-work.