* add helper function to get appropriate DeviceContext

* Run interface return future

* delete unused comments

* implement selectedrows serialize and deserialize

* make serialize/deserialize as global function

* recover send_imp.cc

* delete unused brackets

* fix compile error

* serialize version in LodTensor and SelecetedRows

* fix ci

* fix ci

* Rename API of DeviceContext

Make them as usual names.

* Rename API of DeviceContext

Make them as usual names.

* Fix compile

* Fix compile

* Fix compile

* Fix compile

* Fix compile

* "fix data transform"

* "split into next PR"

* fix bugs

* add KernelTypeToString interface

* cache memory in local scope

* fix typo

* refine trans logic

* "refine kernel registrar"

* "refine registrar with multikey"

* "fix register"

* "refine multikernel register"

* "fix CI"

* "fix CI"

* "fix registry"

* "swtich GPU to CUDA"

* "add register macro test case"

* "fix CI"

Make them as usual names.

Make them as usual names.

Make them as usual names.

* add memory switch mechanism in operator kernel switch

* init data_transform

* complete DataTransform

* fix build error

* add data_transform_test

* add a register test for data_transform_fn

* use function to simulate registration macro

* add register macro

* update test

* clean code

* restore unrelated code

* update data transform test

* generate unique name for REGISTER_DATA_TRANSFORM_FN

* add const

* follow comment

* update KernelTypePair hash function

Cannot get var name recursive since they could be same.

* "fix threadpool style"

* "remove header"

* "add data layout"

* "need kernel registry support"

* "fix data layout"

* "reorder include headers"

* "change enum to enum class"

* "fix CI"

* init kernel hint

* fix typo

* rm unused code

* add include in op_kernel.h

* restore op_kernel since it will be moved to op_kernel_type

* change force_cpu to use_cpu

* fix compilation

* "fix CopyFrom parameters"

* "fix unused Place argument"

* "fixed based on comment"

* implement a simple threadpool

* unlock before cv.notify

* add done function

* add lock with GetAvailable function

* delete done_

* using call_once in GetInstance

* update by comment

* update comment

* enhance unit test for multi threads task

* refine OpKernelKey

* refine codes

* fix code style

* follow comments

* "change operator interface"

* "move devicepool to device_context"

* "fix operator test"

* "fix op_registry Run interface"

* "net op passed. Need to fix nccl multi-Context"

* "add nccl group function"

* "add nccl group function"

* "fix gpu count exceed 32 error"

* "fix recurrent op, nccl op"

* "change the other operators interface with Place"

* "fix typo"

* "fix pybind"

* "fix device in python side"

* "fix pybind failed"

* "add init for test"

* "fix CI"

* add data layout

* fix ci

* "remove GPU Sync Interface"

* "fix typo"

* "fix type cast error"

* "fix related Copy with stream"

* "fix failed tests with DevicePool"

* "fix stupid removed position error"

For input argument with a list of variables, drop_empty_grad is not allowed because it makes the correspondence bewteen a variable and its gradient ambiguous. Use REGISTER_OP_EX to register the op or call InputGrad(?,false) in GradOpDescMaker.