* Support the build for multiple architectures at one cmake command (iOS).

* Update the documentations.

* Chage `IndicateDataType` to `GetKernelType`. Make it easier to
  understand.
* Change `OpKernelKey` to `OpKernelType`
* Make operator developers can customize which kernel the operator will
  use in runtime.

* small fix

* fix comment

* address comment

* small fix

fix nodebook to notebook

* Add English version of Android cross-compiling document

* Add English version of Android cross-compiling document

* Follow comments from Yi-qun and Kavya

* Adding design doc for model average (now called parameter_average)

* Updating title

* Updating image tag

* Updating review comments

* Add documentation of cross-compiling for iOS.

* Correst the typo in documentation of cross-compiling for raspberry pi.

* Set ANDROID_API to 21 when it is specified < 21 for arm64-v8a in build_android.sh.

* Check the input and print the usage in MergeModel.cpp.

* Add cross_compiling_for_raspberry.md

* Update cross_compiling for raspberry pi document

* Some minor edits

* In response to comments from Kavya

* Add the _en suffix

* Add symbolic link from Paddle/CONTRIBUTING.md to doc/howto/dev/contribute_to_paddle_en.md so sphinx can generate the document
* Update CONTRIBUTING.md links so sphinx does not add these links to the TOC
* Removed dev/contribute_to_paddle_cn.md from documentation, since this document is not in sync with Paddle/CONTRIBUTING.md

* Create vlog_guide.md

* Move design/vlog_guide.md into CONTRIBUTE.md

* In response to comments from Yu Yang and Tony

* In response to comments from Luo Tao

* Init

* Stash

* Polish SaveLoadOp

* Fix CI

* Polish code

* Save GPU Tensor

* Stash

* Fix CI

* Removing survey out of the design doc and fixing typos
* Fix Typos

* Extract apply_backward_pass to backward.py

Rename apply_backward_pass to append_backward_ops

* Fix CI

* Update design doc

* "add model format design doc"

* "add restore function"

* "add parse protobuf"

* "move necessary information to saver.proto"

* "format code"

* "add gpu option"

* "add lod info"

* "add saveop python test wrapper"

* "checkpoint reuse save operator"

* "rewrite model format design doc"

* "async support needed"

* "fix run once"

* "fix doc based on comments"

* "refine based on comments"

* "fix based comments"

* "remove persistable flag from framework.proto"

* "add IndicateDataType to restore op"

* "add save test"

* "modify save restore code"

* "modified the restore logic"

* rm checkpoint_op.cc

* rm test_checkpoint_op.py

* "get inputs outputs name from execution context"

* Saving each variable to a independent file

* Fix bugs

* Rewrite save_restore_op_test with new Python framework

* Move `SaveOp` and `RestoreOp` from OpWithKernel to OpBase

* Refine unit test of SaveOp and RestoreOp

* fix compile errorwq