* Remove template parameter for Tensor methods

* Also check the type is correct when data()
* Simplize holder_

* Fix accuracy_op

* Register Code

* init

* unify CopyFrom interface

* fix gpu build error

* fix bug in tensor_py.h

* refine code comments and add TODO list

* fix conflicts in FeedOp and FetchOp

InferShape in Operator should be performed in OperatorBase::Run.

* cond_op, recurrent_op and mnist might be changed in following PR

* remove alias

Fix cycle dependencies, Fix #3583.

* Also simplify pybind implementation by using OperatorBase as holder
  type.

* make stepnet member

* add pybind support

* fix Inputs Outputs

* remove unique_ptr

* finish forward debug

* move namespace rnn to a directory

* move net_op to operators

* Make interface of Operator to `const Scope&`

* the tensor only contains shape and does not hold memory when inferring shape.