* made validation step optional
* added no val model
* val_step can be implemented but not validation_end
* added no val end model
* added tests
* added tests
* remove class
* remove class
* remove class
* remove class
* remove class
* remove class
* remove class
* remove class
* remove class
* remove class
* remove class
* updated docs
* updated docs
* updated test
* updated test
* updated test
* updated test
* updated test
* updated test
* updated test
* updated test
* updated test
* fix pep8