* Make global scope not thread-safe

1. It is no need to make global scope thread-safe, since it will be
invoked in Python main thread.
2. Do not free the global scope when C++ exit. Let the OS free memories,
otherwise, we need to handle the destroy dependencies.

See
https://google.github.io/styleguide/cppguide.html#Static_and_Global_Variables

* Revert "FIX: Release CPU/GPU memory via deleter"

This reverts commit 8f80f5bc.

They are public now

* rm cpp executor_test, rewrite in python later

* remove executor_test code in CMakeList.txt

* Change dataType to data_type

Follow PEP8

* Change name_convention to fit PEP8

* AttributeChecker Better error log and speicalize bool

Since lots of types can be cast to bool

* add FIXME comment

* 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

* fix gometalinter versioning

* stop gometalinter

* Expose Executor to Python

* Follow comments

* 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