remove unused C++ class OpRegistrar

add some basic math functor for SelectedRows

* 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()