Fix Transform API for operator

[reyoung]

• Fix Some problems in paddle::platform::Transform function. #4202

[reyoung]

Since we use static, the Transform will not generate symbols. cc or cu will use different Transform method.

[typhoonzero]

Thought it was the inline keyword that makes Transform not generating symbols?

Markup: https://stackoverflow.com/questions/780730/c-c-static-function-in-header-file-what-does-it-mean

[hedaoyuan]

I do not think this solution is better than #4201. There are two problems with this solution:

1. The Transform interface in .cu.o has a useless cpu code branch.
2. If there are many .cc and .cu are used Transform interface, will compile a lot of duplicate code.

[reyoung]

Maybe we should force inline? I think using a functor is a little bit complex.

[reyoung]

1. We may invoke CPU Transform in .cu file, so that branch is not useless. Also, if we make place as a template parameter, we should use PADDLE_ENFORCE to check the template place is same as the context.GetPlace(). So the if branch will not be removed no matter how we implement Transform.

2. In fact, the BinaryFunctor, UnaryFunctor and the Iterator are the template parameter. Each time we invoke Transform, a new code will be generated. No matter how we implement it.

To make Transform a simple method, not a templated functor, can make users invoke it easily.

It is easier for user write Transform(ctx, input, input+size, output, Functor); than Transform<Place>()(ctx, input, input+size, output, Functor);.

[qingqing01]

not_test_check_grad -> test_check_grad

[reyoung]

Done.