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.

Test plan

TODO: add test for this with nn.Linear/LoraLinear, where we can see that qdata/scale (zero not stored) are all in PTD file after quantization, and no weights are in PTE file.

[pytorch-bot]

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