Rewrite mixed_vector.h

Remove 'numel' checker in compare op

* initial commit

* minor fix

* remove redundency

* address comments

Delete "API" section from "Documentation"

create optimize block in pserver program

Fix the bias of fc when num_flatten_dims is not 1 in fluid layers

Add python wrapper for layer normalization.

* Save the inference model in Python example of recommender_system.

* Add infer() in Python unittest recommender_system.

* Add C++ inference unittest of recommender_system.

Simplify the cmake of inference

refine error msg and add unit tests for cpp reader

Optimize return all optimize and related ops

Refine Channel's unit test

Fix CI hung

Refine distribute transpiler

GPU implementation for layer normalization operator.

* Refine load

* Address review comments: round 1

* Make API consistent with python-save/load

* Add another unit test

* Remove commented function

* Fix GPU bug

* Address review comments

* Modify wrt PR 8147

* Fix filenames for combined case

* Fix typo

* Address review comments: round 2

* Unify TestInference by keeping default param in template

* Address review comment

* Fix spacing

Add Inference example and unit test for rnn_encoder_decoder