For input argument with a list of variables, drop_empty_grad is not allowed because it makes the correspondence bewteen a variable and its gradient ambiguous. Use REGISTER_OP_EX to register the op or call InputGrad(?,false) in GradOpDescMaker.

Adding documentation for the operators: lod_tensor_to_array , array_to_lod_tensor, create_array, increment (#6807)

* Adding operator assignment

* Adding a prototype for documentation in layers

* small update to re-run Travis

* Removing file from another PR

* Small typo

* Adding documentation for the operators: lod_tensor_to_array , array_to_lod_tensor, create_array, increment

* Fixing indentation issue

* Fixed datatype of input variables

* Polish API docs for assign layer

* Polishing the API docs for concat and assign layer

* Writeup for array write layer

* Fixed the type

fix pip install page 404 links

Rename fill_zeros_like_op's output

* Remove unnecessary reshape in ColwiseSum

Speed up 12s -> 10s.

* Hand write ColwiseAdd in CPU

* add design doc on keys of operator kernel type

* follow comments

Add dilation for conv_trans_op

* Rename XXDescBind --> XXDesc

* Fix Compile

* Add libprotobuf-lite.a when install.

* Fix protobuf.cmake

* Bug fix

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