* 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.

* "replace enum with string"

* "fix layers"

Set gradient's data type based on its forward variable

* Init commit

* Make executor use ProgramDescBind

* Change Attribute from BlockDesc to BlockDescBind

* Since we will get the program desc in RNN, just BlockDesc is not
  enough.

* add shareLod

* fix sequence_conv grad infershape

1. Change some `auto` to `auto*`
2. Change `Tensor` to `LoDTensor`

* refine evaluator op types

* update

* follow comments

* update

* fix v2 mnist case

* fix v2 mnist case

* update

* update

Use posix_memalign to allocate aligned memory, since some SIMD instructions require the alignment of memory accesses.

* refine evaluator op types

* update

* follow comments

* update

* fix v2 mnist case

* fix v2 mnist case

* update

* update

* Complete accuray layer

* Fix error

* Fix error

* Add 'accuracy' to __all__

* update

* Fix Type error

* Fix error

* Refine unit tests

* Fix an unit test error

* reimplement pow operator

* add pow_grad operator

* fix code style

* fix build error

* fix op_test bug

* revert pow operator

* add FIXME comment

* remove unused code

* fix cmake file

* fix build error

* improve unique_name, uniq id is related to prefix

* fix join

* Fix Type error

* Fix error

* Fix top_k_op GPU code data type

* add init_gflags interface

* refine code

* follow comments

* Fix Type error

* Fix error