* 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

move ENFORCE position

enhance DataFeeder

Polish the doc of cross_entropy

* 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"

Add conv2d_python doc

* fix bugs

refine CMakeLists.txt when add op need DEPS

Fix two docs

* add KernelTypeToString interface

* cache memory in local scope

* fix typo

* refine trans logic

[WIP] new backward

Optimize the rowwise add function.

fix dist train trainspiler bugs

* "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"

Refine drop_out_op and batch_norm

* add memory switch mechanism in operator kernel switch