* move net_design to framework

* change CreateNet result to unique_ptr

* rename "ScratchNet" -> "PlainNet"

* add three methods to NetBase

* add NetBuilder

* add InferShape to NetBuilder.Run

* rename ApplyGradient, ApplyOptimizer -> AddGradientOps, AddOptimiz

* rename PlainNet::CreateNet -> BuildNet

* add Error and other rename actions

OpProto is a proto message that helps 3rd-party language bindings, e.g.
`Python`, to generate operator creation methods. The operator creation
method is the low-level API for 3rd-party language bindings. Op creation
methods take the user's input in that language, and convert users inputs
into `OpDesc` message, then passing that `OpDesc` message to Paddle's
C++ core and create an operator.

* A separated `attr_type.proto` is added, because that file wound
  be included by `op_desc.proto` in future.

Make user can check what type is in variable