Change the way a module callback resolves the module name

When we are traversing the forward path of a graph, by invoking each
module's forward_hook callback, we sometimes want to know the full
name of the module.
Previously, to infer the module name, we looked up the name of self.weight
parameter and used that to get the module name.
In PyTorch 0.4 we can directly look up the module name using
model_find_module_name.