feat: skip NVFP4 compression on meta-device tensors

[kylesayrs]

Summary

• Adds _skip_meta_device decorator to NVFP4PackedCompressor.compress() that short-circuits quantize+pack when the weight tensor is on the meta device
• Constructs correctly-shaped meta placeholders (weight_packed as uint8, weight_scale cast to scale_dtype) instead of running actual computation
• Enables resumable model-free PTQ runs where already-processed shards are loaded as meta tensors for weight map / total_size accounting

Companion PR: llm-compressor (TBD) adds the resume/skip pathway in process_file()

Test plan

• Verify NVFP4 compression still works normally (non-meta tensors)
• Verify meta-device tensors produce correct placeholder shapes
• Verify MXFP4 inherits the decorator correctly (group_size=32 path)

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[mergify]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @kylesayrs.

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