Refactor DDim's product() and add slice_ddim()

[JiayiFeng]

1. Refactor DDim's product() to make it more efficiently.
2. Add slice_ddim().

fix #2880

[luotao1]

加下注释吧。

[JiayiFeng]

done

[QiJune]

Do we have to check the value of begin and end here?
such as

begin <= end;
begin>=1;

[JiayiFeng]

We do these checks in SliceVectorizeVisitor.

SliceVectorizeVisitor(std::vector<int>& v, int b, int e)
      : vector(v), begin(b), end(e) {
    PADDLE_ENFORCE(begin < end,
                   "Begin index must be less than end index in ddim slice.");
    PADDLE_ENFORCE(begin >= 0,
                   "Begin index can't be less than zero in ddim slice.");
  }

and

void operator()(const Dim<1>& dim) {
    PADDLE_ENFORCE(end == 1, "End index in ddim slice is out of bound.");
    vector.push_back(dim.head);
  }

[QiJune]

I know that a Dim<D> variable can be passed to a DDim variable.
But, how a DDim varibale passed to a Dim<D> variable?

[JiayiFeng]

See https://github.com/Canpio/Paddle/wiki/Majel:-From-Allocation-to-DArray
In the last section, I show how DArray is casted into Array in boost::apply_visitor. The principle is also suitable for DDim and Dim.

[QiJune]

LGTM

[QiJune]

This implementation is a little tricky. And all implementation related with dim is tricky.
We may consider remove dim later and just use a small vector.