[vllm] fix: ignore MoE router layers for FP8 quantization

[zpqiu]

What does this PR do?

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• Add logic to ignore MoE router layers (model.layers.{i}.mlp.gate) during FP8 quantization
• Remove unused FP8_BLOCK_QUANT_KWARGS constant from vllm_fp8_utils.py

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[sharonyu-115]

For reference, here is the before and after comparison result:

Orange: BF16 rollout + BF16 train
Purple: FP8 rollout (with router quantization) + BF16 train
Red: FP8 rollout (without router quantization) + BF16 train - much lower mismatch KL