Delete VarDesc in Variable

[tonyyang-svail]

https://github.com/PaddlePaddle/tape/blob/develop/src/variable.h#L84

Tensor and SelectedRow already have dim and dtype, doesn't need duplicated information VarDesc.

The problem is:

1. Without OpDesc, InferShape only exists in OperatorKernel. So maybe only OperatorWithKernel can be added to tape.
2. InferVarType only exists in the Desc level. There is no runtime InferVarType.