'VarDesc' needs the support of holding multiple LoDTensorsDesc

[JiayiFeng]

So far our VarDesc can only hold one LoDTensorDesc. Although a LoDTensorArray has more than one LoDTensor in the runtime, all the LoDTensors share the same shape and data type, so a single LoDTensorArray is enough to describe them.

However, since we are now trying to implement readers in C++, the support of multiple LoDTensorDesc in one VarDesc becomes necessary. In our design, a reader is held by a Variable, and of course, it can yield more than one LoDTensor once. These LoDTenosrs are likely to have distinct shapes, data types and LoDs. To describe all of them during the compile time, our VarDesc must be able to hold more than one LoDTensorDesc.