Why vllm store fp8 kv cache as uint8_t, but not torch.float8_e4m3fn or torch.float8_e5m2?

[cyLi-Tiger]

Here, key_states and value_states are being converted from BFloat16 (bf16) to uint8_t. Will this conversion cause performance issues due to the difference in range between these types? Additionally, how should we handle negative values during this conversion?