op helper function for generating grad var's name

[Superjomn]

No description provided.

[JiayiFeng]

I think renaming GRAD_VAR_SUFFIX and making it return var + grad_suffix directly may be a better choice? Introducing a new namespace make things more complex.

[Superjomn]

good ideas. but there are some strange GRAD_VAR_SUFFIX usages, such as

https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/framework/backward.cc#L138
https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/framework/backward.cc#L150

It seems that GRAD_VAR_SUFFIX is necessary for these usages, GenGradName is added to hide the details of grad var's name from developers.
@Canpio

The namespace op_helpers is added, because operators needs many helper functions, GenGradName is the first, many other ones should be offered, such some helpers to help these simple check:

PADDLE_ENFORCE(ctx.InputSize() == 2,
               "Input size of OnehotCrossEntropyOp must be two");
PADDLE_ENFORCE(ctx.OutputSize() == 1,
               "Output size of OnehotCrossEntropyOp must be one");

may need a helper

void EnforceInputsOutputsCountMatchProto();

PADDLE_ENFORCE(ctx.InputVar(0) != nullptr && ctx.InputVar(1) != nullptr,
               "Inputs of OnehotCrossEntropyOp must all be set");
PADDLE_ENFORCE(ctx.OutputVar(0) != nullptr,
               "Outputs of OnehotCrossEntropyOp must all be set");

may need a helper

void EnforceVarsValid(xxx);

PADDLE_ENFORCE(ctx.Input<Tensor>(0)->dims() == ctx.Input<Tensor>(1)->dims(),
               "Two input of Add Op's dimension must be same.");

may need a helper

void EnforceShapeMatches(const std::vector<std::string>& var_names);

So, I will create an op_helpers directory and keep this namespace.
@wangkuiyi

[reyoung]

@Superjom @Canpio

Rename GRAD_VAR_SUFFIX --> GenGradName().

string GenGradName(string name) {
  return name + "@GRAD";
}

and suffix could be GenGradName("");

[Superjomn]

work, but GenGradName("") is a little weird, but it can help to hide GRAD_VAR_SUFFIX. I will follow this idea first.
@reyoung

[wangkuiyi]

为什么需要放在 op_helpers 这个namespace里呢？看上去可以在 framework 里就好了。

根据 https://google.github.io/styleguide/cppguide.html#Namespace_Names ，如果要引入 op_helpers，需要创建一个子目录叫 op_helpers。

[Superjomn]

get it.

[wangkuiyi]

需要为这么一个函数创建一个namespace吗？是很快就会有其他函数被加入进来吗？如果不是，就放到 framework namespace里是不是“最简单”的做法？

[Superjomn]

对，应该很快会加op inputs/outputs 一些check的helper function，简化一部分 InferShape 里的写法