paddle::framework::Tensor design issues

[wangkuiyi]

1. set_dims ==> Resize
2. ShareDataFrom ==> ShareDataWith
3. The following code snippet in ShareDataFrom

    std::vector<int> d = vectorize(dims_);
    int base = 1;
    for (size_t i = 1; i < d.size(); ++i) {
      base *= d[i];
    }

   looks like

   product(dims_);

4. Document Tensor::offset_ as for supporting slicing.
5. CheckDims ==> MustNotEmpty
6. typename PType ==> typename Place
7. Move class Deleter to paddle::memory.
8. Remove T* raw_data() const.
9. In SharedDataWith, replace

    holder_ = src.holder_;
    Resize(src.dims());
    start_ = src.start_;

   by

   *this = src;

10. DDim dims() const ==> const DDim& dims() const