[ExecuTorch][Llama][ez] Remove unwrap_tensor_subclass from 4w quantization path

[SS-JIA]

Stack from ghstack (oldest at bottom):

• -> [ExecuTorch][Llama][ez] Remove unwrap_tensor_subclass from 4w quantization path #18957

D100066455 changed the Llama export pipeline to quantize weights in the checkpoint dtype (typically bfloat16) before casting to the computation dtype (fp32). This introduced a regression for Vulkan 4w export: dequantize_affine ops produced bfloat16 outputs, which Vulkan doesn't support, causing the graph to be split into multiple partitions. When sym_constrain_range_for_size constraint nodes were partitioned into a different delegate than the _local_scalar_dense + slice_copy ops they constrain, ExportPass re-tracing (in ConvertToLinearPass, SpecPropPass, etc.) would crash with GuardOnDataDependentSymNode: Could not guard on data-dependent expression u539 < 0.

The root cause is unwrap_tensor_subclass(). This function decomposes IntxUnpackedToInt8Tensor into plain tensors via torch.nn.utils.parametrize, capturing the subclass's metadata — including its dtype attribute (which controls dequantize_affine's output dtype) — as a frozen snapshot in UnwrapTensorSubclass.rebuild_stack. A subsequent model.to(dtype=fp32) casts the plain tensors but cannot update the frozen metadata, so dequantize_affine continues to output bfloat16.

unwrap_tensor_subclass() was originally needed as a workaround because torch.export did not support tensor subclasses natively. This is no longer the case — the 8da4w path already works without it (using the same IntxUnpackedToInt8Tensor subclass), and torch.export traces through the subclass correctly. Removing it makes 4w consistent with 8da4w and avoids the metadata-freezing issue entirely.

This change was authored with Claude.

Differential Revision: D101252037

[pytorch-bot]

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• [CI[B200] Smoke test encounters CUDA Unknown error for dgxb200-03 and dgxb200-04

❌ 2 New Failures, 2 Unrelated Failures

As of commit 30e9d98 with merge base a43675c ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model
• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model

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

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

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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