Do not use ctor

* Reduce line of codes.
* We can use virtual function for Maker now.
* The implementation does not care what maker holds, it is easier to
refactor later.

* fix mac build error

* override GetExpectedKernelType for cast op

* fix typo

* add cuda unittest

* Rename XXDescBind --> XXDesc

* Fix Compile

* Move framework.proto to proto namespace

* Fix compile

* Fix compile

* Fix Compile

Also add `elemwise_sub/mul/abs/clip` layers

There are mainly following fixes:

- take `DeviceContext` as the template parameter of math functors and OpKernel instead of `Place`
- remove `eigen_device` interface in base class  `DeviceContext`
- remove `GetEigenDevice` interface in `ExecutionContext` and base class `DeviceContext`
- remove unused `platform::EigenDeviceConverter`
- rename `REGISTER_OP_GPU_KERNEL` to `REGISTER_OP_CUDA_KERNEL`
- rename `USE_GPU_ONLY_OP` to `USE_CUDA_ONLY_OP`

* Change all `data_type` in Python to `dtype`

* Change `date_type` in C++ to `dtype`

* Refine

* Adding the doc format for AdaDelta

* Updating the documentation for Adagrad, Adam and Adamax

* Updating the auc op

* Fix review comments

* Updating doc for Batch Norm

* Updating the cast op

* Updating the clip op

* Fixing review comment

* Fixing review comment:

* Small change to restart PR_CI

* Cast Operator

Cast input variable to other data type

* Fix compile error

* Add cast op

* Follow comments