[Bugfix] Fix kv_cache_dtype=fp8 without scales for FP8 checkpoints

[mgoin]

FIX #6738

Enabling kv_cache_dtype=fp8 quantization without scales in an FP8 model checkpoint was broken since the default k/v_scale wasn't saved as a float.

On main you would see this error if enabling fp8 kv cache with an FP8 model:

> vllm serve neuralmagic/Qwen2-0.5B-Instruct-FP8 --enforce-eager --kv-cache-dtype=fp8

[rank0]:   File "/home/mgoin/code/vllm-ct/vllm/model_executor/model_loader/loader.py", line 294, in load_model
[rank0]:     quant_method.process_weights_after_loading(module)
[rank0]:   File "/home/mgoin/code/vllm-ct/vllm/model_executor/layers/quantization/kv_cache.py", line 65, in process_weights_after_loading
[rank0]:     raise ValueError("Only support per-tensor scaling factor "
[rank0]: ValueError: Only support per-tensor scaling factor for fp8 KV cache

[github-actions]

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