FP8 PTQ With Physical Dependency 0515

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描述：定义新的FP8 observer，目前只支持e4m3格式
功能：统计量化对象的abs_max，并根据abs_max和fp8格式的取值范围计算scale
PaddleSlim设计逻辑：
paddleslim/quant/observers/fp8uniform.py
定义FP8UniformObserverLayer类用以统计量化对象的abs_max以及根据用户选择的要量化到的FP8类型设置正确的量化区间，并最终计算出scale，方便FP8LinearQuanterDequanter进行量化和反量化操作
paddleslim/quant/observers/init.py
将FP8UniformObserver添加到__init__中方便外部调用

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