The learning rate function is advanced by the ML models. Moved reference
to parent from the Gradient subclasses to the root class.
	modified:   ml/linear_regression.cpp
	modified:   ml/linear_regression.h
	modified:   ml/ml_model.cpp
	modified:   ml/ml_model.h
	modified:   ml/neural_net.cpp
	modified:   ml/neural_net.h