* Initial draft for SGD BG16 kernel.

* Unit tests for SGD with BF16 data type.

* Add VLOG message to SGD BF16 op CPU kernel.

* Enhance error messages and error types.

* Refactor SGD op kernels to leverage some common code.

* Make easier to add new kerne invoke code.

* Fix SGD op kernel for sparse grad.

* Unify quotes style.

* Fix error for ROCM compilation.

* Use specialized PADDLE_ENFORCE_xx functions.

* fix sgd/momentum/dpsgd/rmsprop error message

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* wip simplify operator framework

* wip

* wip

* done test=develop

* clean test=develop

* fix test=develop

* fix deps test=develop

* fix cpu build test=develop

* fix tensorrt build test=develop

* fix tests test=develop

* fix test=develop

* fix cpu build test=develop

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Fix code to support cpplint syntax check

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Optimize selected rows for dist lookup table with rwlock 

Fix sparse update memory error for distributed training

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`

* Changing learning rate from attribute to input(float)
* Removing obsolete code

Fix #3902