* Feed/Fetch op just plain operator, not a OpWithKernel
* Do not register OpInfoMaker since Feed/Fetch will never be
  configured by users
* Feed/Fetch op has empty gradient
* Feed/Fetch op do not hard code `feed_variable`, `fetch_variable` as
  its input and output, make it as a plain Operator input/output

 add SQRT/LAST/FIRST strategy for Seqpool

Ensure name is not set in bias_attr in gru_step_naive_layer

Fix bug in computing output size of IdentityOffsetProjection

* fix gometalinter versioning

* stop gometalinter

remove duplicated doc/tutorials, and rename tutorials to v1_api_tutorials

lambdaCost layer's bug fix

* add sync_with_cpp to Python Program and Block

* sync vars and ops in block from cpp

* optimize code and add some comment

* add more check for sync

* Expose Executor to Python

* Follow comments

Backward return map

Merge mkldnn output grad

change: NewVar() to Var()

* Final step of backward, return a map from param_name to grad

* Complete the final step of backward

Return the param_name to grad_info

* add target to Backward, generate var in block when call backward

* modify backward_test

* fix executor_test

* set var desc default type to LOD_TENSOR

* update backward_test

* insert loss in the top level of backward

* create grad vars for all blocks in current program

* optimize code

* update test_program.py

* only create var for newly create blocks when backward