fix batch_norm parameter share

Design doc: operator based parameter server.

Fix name in test_lookup_table

Add axis for `mul_op` and `rowwise_add_op`

rewrite backward graph

Update functions_operators_layers.md

Fix android-16 compile.

Fix error on TravisCI

It seems that Travis change the running image. The /usr/local/ is
not writable in new images. Use `sudo` for pip to resolve this.

* init add

* add topk op

* someupdate

* fix style check

* add test py file

* update top k cuda kernel

* follow comments

* remove debug print

* fix casting error

* fix casting error

* fix casting error

* fix rename bug...

* fix travis

Fix warning log in switch order layer

Refine names and doc of some operators.

fix im2col_test.cc on cpu only machine

Remove build_android task from travis temporarily

Remove build_android task from travis temporarily to avoid failure on travis because of the cache of third_party.

Enable the use of clang to cross-compile for Android

Add im2col functor

Neon Depthwise Convolution Transpose Function

Add switch order layer for FCN model

add VarDesc design