[Accuracy diff No.16] Fix accuracy diff for paddle.cumsum、paddle.logcumsumexp API

[cangtianhuang]

PR Category

Operator Mechanism

PR Types

Improvements

Description

错误定位：

1. ThrustCumsumKernel 自身的精度误差就极大
2. fp32 下大 tensor 拥有累计误差

解决方案：

1. 删除 ThrustCumsumKernel 分支处理，继续使用 CUDA cub 计算
2. BlockPrefixCallbackOp 采用 Kahan 算法，参考：https://en.wikipedia.org/wiki/Kahan_summation_algorithm
3. 将 LogAddExp 的 Identity 从 std::numeric_limits<T>::lowest() 改为 -std::numeric_limits<T>::infinity() ，使其在理论上更准确

其他修改：

1. 其他计算中的 lowest 改为 -inf，确保一致： paddle/phi/kernels/cpu/cum_kernel.cc 、 test/legacy_test/test_logcumsumexp_op.py 、 paddle/fluid/primitive/decomp_rule/decomp_vjp/details.h
2. 整理 paddle/phi/kernels/impl/logcumsumexp_grad_impl.h ，添加注释

测试：

Pcard-85711

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[cangtianhuang]

/re-run all-failed

[cangtianhuang]

/re-run all-failed

[codecov-commenter]

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