Move and Hide PyVisitor Mutator

[tqchen]

The PyVisitor/Mutator are only used in the python side and as long as we have packed func registration we are fine. To minimize the public interface exposure, let us consider move and hide PyVisitor/Mutator into a cc file instead

[tqchen]

cc @LeshengJin @YuchenJin

[junrushao]

Sure thing. I don’t think it’s an architectural decision though

[tqchen]

right, this is nor strictly architectural, but more of an organizational issue :)