Add flatten op interface and enhance APIs about detection to support variable-length image. (#12422)

* add flatten api&enhance detection api

* unify shape_op data type

* update API.spec

Make cross_entropy_op supporting tensor

Polish API of exception holder

test and doc IR Graph

disable anakin temporary

add export_for_deployment flag to save_inference_model

Add tensorrt softmax

* make inference_anakin_test SERIAL

* add anakin compiler from github source code

* fix inference_lib_dist error

* add comment

* update anakin.cmake

* fix anakin-NOTFOUND compiler error

* modify the anakin_model download dir

Revert "Refine elementwise_add op"

* hide misc APIs

* update

* fix transformer test

* update API.spec

* fix dist train with rmsprop

* add rmsprop transpiler test

* update by comment

* fix the numpy version has to be lower than 1.15

* Add TODO comments

Fix 'softmax_with_cross_entropy_op' dependency error

Refine layer_norm op

* Fix infer shape of crop op.
* Speed crop op.

fix wmt14 download error

polish dist unitest

fix missing macro condition

Refine elementwise_add op

code clean up and renaming

higher review requirement for adding new APIs

Add py3 env for manylinux

add RPCServerProfiler, replace listen and serv optimizer

Timeline support pure cpu