Add the LinearChainCrf operator.

* 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 backward implementation for LSTM operator. 

Allocate aligned memory by posix_memalign.

* "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`

* AddBiasOp does not care num_flatten_dims

* Add comments

* "modify layers.py"

* "fix pool interface"

* "add export type to layers"

* "fix based on comment"

* refine evaluator op types

* update

* follow comments

* update

* fix v2 mnist case

* fix v2 mnist case

* update

* update

* AddBiasOp does not care num_flatten_dims

* Add comments

* "modify layers.py"

* "fix pool interface"

* "add export type to layers"

* "fix based on comment"

correct the index of cluster_train_cn/en.md

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