Decompose after export in export_llama

[lucylq]

Summary

unwrap_tensor_subclass was not unwrapping nested lora linears. This meant qdata/scale/zero were bundled together in the subclass, and separated at run decompositions inside to_edge_transform_and_lower. This is after nodes are tagged, meaning that the scales were not tagged, and remained in the PTE file after the rest of the weights were moved to a PTD file.

It's recommended to move away from unwrap_tensor_subclass and rely on export + decomps. This PR adds a decomp after exporting in export_llama, and removes cases of unwrap_tensor_subclass.

TODO: remove all cases of unwrap_tensor_subclass in ET.

Test plan

Add check that quantized weights are in PTD file (not PTE file) after quantization. This is a simple check, nested linears seem to be the real issue that decomposing resolves. TODO to add a test for that (probably e2e test with stories in subsequent PR)

 python -m unittest executorch.backends.xnnpack.test.passes.test_propagate_custom_meta_pass

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15951

• 📄 Preview Python docs built from this PR

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

❗ 1 Active SEVs

There are 1 currently active SEVs. If your PR is affected, please view them below:

• Dr CI is temporarily not working due to API fairewall

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.

[meta-codesync]

@lucylq has imported this pull request. If you are a Meta employee, you can view this in D87826410.

[metascroy]

LGTM! Address any failing CI tests before landing.