enable merge grad unit test

add config helper for dot-product attention.

* 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

Complete infer_var_type

* rm cpp executor_test, rewrite in python later

* remove executor_test code in CMakeList.txt

export SelectedRows to Python

* Implement FC layer with helper

* Update LayerHelper

* Add debug string for Python ProtoBuf

and Rename `Sync` to `Flush`

* Add check of ProtoBuf initialization

* Layer wrapper for FC

* Fix unittest

* Fix CI

* Add code generator

* AttributeChecker Better error log and speicalize bool

Since lots of types can be cast to bool

* Complete mlp, fit_a_line

export feed/fetch variable method to Python

* Change dataType to data_type

Follow PEP8

* Change name_convention to fit PEP8

* Update design doc for Python Layer

* Update document

* AttributeChecker Better error log and speicalize bool

Since lots of types can be cast to bool

* add FIXME comment

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

and add activation in unit tests

 add SQRT/LAST/FIRST strategy for Seqpool