[Bugfix] Restore is_sym guard for zp in GPTQ/CT MoE to fix symmetric quant regression

[yuwenzho]

Purpose

Fix a regression introduced by #43409 (comment) that broke symmetric GPTQ MoE quants (e.g. Autoround) on NVIDIA GPUs.

#43409 removed the if not self.quant_config.is_sym else None guard in AutoGPTQMoEMethod.get_fused_moe_quant_config so that CPU could pass synthesized zero points for symmetric models. However, this caused symmetric GPU quants (Autoround and standard GPTQ with is_sym=True) to pass meaningless qzeros tensors to the Marlin kernel, producing incorrect results.

The same pattern existed in CompressedTensorsWNA16MarlinMoEMethod.process_weights_after_loading, where the if not self.symmetric: guard was absent, risking incorrect zp registration for non-CPU symmetric backends.

Fix

auto_gptq.py and compressed_tensors_moe_wna16_marlin.py: Restore the is_sym guard with an explicit CPU exception

Test Plan

CPU tests:

• GPTQ W4A16 INT4 MoE: Qwen/Qwen3-30B-A3B-GPTQ-Int4 passed
• Compressed-tensor W4A16 INT4 MoE: RedHatAI/Qwen3-30B-A3B-quantized.w4a16 passed

Test Result

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