gather function only

Random op

A better error message for gradient checker

Remove unnecessary C++ operator test

They are tested in Python

* Give which parameter, which element are wrong. And what max_diff is.

Make const variables in operator.h fit google style

Use ostream << operator to get to_string

* No POD instance is forbidden in global scope.

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

Merge pull request #3334 from reyoung/feature/ignore_some_files_which_will_generated_while_compiling

Ignore compile generated files for git

* Make `PADDLE_ENFORCE_EQ` supports custom class, like DDim

* merge InferShapeContext and OperatorContext

* OperatorBase& instead of OperatorBase*

Implement trainer init parameters election with etcd

* add lodtensor

* add reshape of lod

* add details

* rename Elements/Levels

* size_t and vector reserve

* add details

* add const& std::shared_ptr

* add lod_tensor_impl.h

* remove a shared_ptr

Refactorize enforce_test.cc