Allow Unsloth Dynamic 4bit BnB quants to work

[danielhanchen]

This PR allows vLLM to skip applying bitsandbytes quantization to certain layers, and leave them in 16bit. This will for now work only on skipped modules specified inside of llm_int8_skip_modules

For example https://huggingface.co/unsloth/DeepSeek-R1-Distill-Llama-8B-unsloth-bnb-4bit/blob/main/config.json has

"llm_int8_skip_modules": [
      "lm_head",
      "multi_modal_projector",
      "merger",
      "modality_projection",
      "model.layers.1.mlp"
],

will skip quantizing model.layers.1.mlp. Tagging @mgoin ! :)