Created by: wangkuiyi

As stated in https://github.com/PaddlePaddle/Paddle/pull/3186#pullrequestreview-53925149, we have code snippets like

class Something {
  constexpr T kLOG_THRESHOLD() const { return static_cast<T>(1e-20); }
};

The snippet has two problems -- 1. the naming violates code style, 2. a constant should be define with specific type as it is obvious that 1e-20 cannot be casted into any integral type.