[Bugfix] Change kv scaling factor by param json on nvidia gpu

[bjmsong]

According to the docs, when the checkpoint is in FP16, the kv-cache scaling factors can be loaded via --quantization-param-path.
With the help of this change, the accuracy will increase.

	kv_cache_dtype	quantization_param_path	gsm8k
Before	fp8	No	0.22
	fp8	Yes	0.22
After	fp8	No	0.22
	fp8	Yes	0.228

Reproduce script:

lm_eval --model vllm \
  --model_args pretrained="{Llama-2-7b-chat-hf}",add_bos_token=true,kv_cache_dtype=fp8 \
  --tasks gsm8k \
  --num_fewshot 5 \
  --limit 250 \
  --batch_size 'auto'
  
lm_eval --model vllm \
  --model_args pretrained="{Llama-2-7b-chat-hf}",add_bos_token=true,kv_cache_dtype=fp8,quantization_param_path="{kv_cache_scales.json}" \
  --tasks gsm8k \
  --num_fewshot 5 \
  --limit 250 \
  --batch_size 'auto'

Caution: lm_eval doesn't support the kv_cache_dtype params, I have pulled a PR here

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[mgoin]

As the comments mention we'd like to move away from the quantization_param_path interface in the future as we also support kv cache quantization through quantization backends like compressed-tensors, but this change makes sense as a bugfix. Thank you