Slight change to Tensor methods

[wangkuiyi]

Slight change to some Tensor methods from

  inline void Resize(const DDim& dims);
  inline void ShareDataWith(const Tensor& src);

to

  inline Tensor& Resize(const DDim& dims);
  inline Tensor& ShareDataWith(const Tensor& src);

so that we can write

return LODTensor(lod_.Slice(...)).ShareDataWith(*this);

and

tensor.Resize(new_dim).mutable_data<float>(...);