Kernel hint design

Refine elementwiseAdd and im2col

Correct the dropout_op's computation in test

* Polishing the embedding layer and the fc layer documentation
* Addressing code review feedback

* Updating the design doc of Fluid

* Organizing the operator documentation

* Adding a proposed format for operator documentation

* Adding more details to the format

* Add design of switching kernel

* Follow comments

Add python wrapper for reduce_sum

* multi cpu design

* update

* multi cpu executor to executor

* add graph converting

* use parallel operator to execute blocks with multi threads

* use auto-transpiler

* use auto-transpiler

* update

* update graph

reopen linkchecker for checking broken links in websites

Add separable convolution

* Move framework.proto to proto namespace

* Fix compile

* Fix compile

* Fix Compile

Fix 404 in k8s single training

Expose const value

fix some doc errors

* Support Clip in param_attr

* Fix the order of clip & regular

Regular is not need to be clipped